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[flffl! iHimnnnlin in nlft n tiiiiiiitiiitii liiiitii inn I miiiiiii iiHitin 




Class ^, ^11. 

Book _^ 

GqpghtN?._ 

COPYRIGHT DEPOSIT. 



The D. Van Nostrand Company 

intend this booK to be sold to the Public 
at the advertised price, and supply it to 
the Trade on terms which will not allow 
of discount. 



SUPERHEAT, SUPERHEATING, 



AND 



THEIR CONTROL 



BY 

WILLIAM H. BOOTH 

M. Am. Soc. C..E. 

Formerly of The Manchester Steam-users' Association 

Lecturer to the Royal School of Military Engineering, Chatham 

Author of "Liquid Fuel and its Combustion," "Steam-pipes" 

'■ Water Softening and Treatment," "Smoke Frevention and Fuel Economy* 




NEW YORK 

D. VAN NOSTRAND COMPANY 

23 Murray and 27 Warren Streets 
1907 



J 



< 5 V 



^ 

& 



LIBRARY of CONGRESS 

two Gooles Received 

OCT 19 *30f 

Copyright Entry 

QU ft '?<7 

CLASS A XXC, NO, 

/f 33 

COPY B. 



COPYKIGHT, 1907 
BY 

D. VAN NOSTRAND COMPANY 



A 






f 






ROBERT DRUMMOND COMPANY, PRINTERS, NKW YORK 






TO THE MANY FRIENDS IN THE 
STATES AND CANADA WHO HAVE 
MADE MY VISIT A PLEASURE AND 
PLEASANT MEMORY. 



PREFACE 



At this date no apology is needed for a small book on the 
subject of superheat and superheaters, because there should 
never be required an apology for writing a book upon any 
process in the arts and manufactures which will enable a con- 
sumer of fuel to consume less fuel without incurring great 
expense, fuel being the life of most of our modern industries, 
and, moreover, being a natural product the further manu- 
facture of which in Nature's laboratory is not proceeding, 
at least in those areas from which it is at present being ex- 
tracted, with great rapidity. Since the use of superheated 
steam is essential to the best economy of fuel it is necessary 
that the process of superheating should be one that commends 
itself to steam-users, to the men on whom we are calling to 
practise economies. It is therefore essential that superheating 
should be an operation that can be carried on with a minimum 
of anxiety and risk, that it should be at least as regular as 
the other stages of steam production. All processes that 
have passed through a period of experimentation are very apt 
to acquire the character of uncertainty and non-reliableness. 
Superheating is no exception to this, for it has suffered not 
only because of its own early troubles, but it has suffered from 
causes entirely external to itself which have long ceased to be 
operative and never had any real connection with the process 



VI PREFACE 

of superheating. These external causes, combined perhaps with 
other side issues, were powerful in delaying all real progress in 
the art for forty years. 

During those forty years, while steam-engineering has 
made progress, it has found its field of usefulness gradually 
invaded from various points. The gas-engine has made large 
inroads. Much yet remains to be done towards perfecting the 
gas-engine until its competition shall become still more keen. 
The Diesel and other liquid-fuel engines have come into consid- 
erable use. The employment of electrical transmission of power 
has enabled falls of water to be utilized that have hitherto 
been impracticable. The thermal efficiency of the gas-producer 
and gas-engine is superior to that of the steam-boiler and 
steam-engine, but the steam-engine is thus far greatly superior, 
as a machine, to the gas or any other internal-combustion engine. 
But economy of fuel is so important that every advance in the 
internal-combustion engine will bring it in a winner in the 
debatable borderland in which some trivial circumstance will 
fix the decision for or against any particular type of prime 
mover. 

For these and other reasons, then, the steam-engineer must 
pay intelligent and careful attention to the art and science of 
superheating. 

In this book the reader will not seek for illustrations of 
the many superheaters on the market. For these he is referred 
to makers. The cuts in this book are intended only to show 
examples of types and are few in number. The author prefers 
to indicate principles rather than to write a book of the catalog- 
compilation type. While his personal prejudices are in favor 
of the fully water-controlled type, he also recognizes that this 
type must necessarily be more costly. Superheating is a 
modern recrudescence. Its apparatus may vary, and would-be 
users should first understand the possibilities and principles 



PRE i« ACE vu 

of superheat and may then examine how far makers can offer 
apparatus that will fill their requirements. The author of 
necessity cannot advocate an) special apparatus, and to meet 
the inevitable critic would say at once that the examples em- 
ployed for illustration are simply such as he found most avail- 
able. 

To his publishers the author would record his grateful 
acknowledgments. 

William H. Booth. 

2 Queen Anne's Gate, Westminster, 

AND 

220 West 57th Street, New York. 



CONTENTS 



CHAPTER I 

PAGE 

The Past History of Superheat j 

The Cornish Engineers. Modern Theory of the Steam-engine. 

CHAPTER II 

Specific Heat of Materials . . : 7 

Steam Expansion Curves. Action of Steam in a Cylinder. 
The Elementary Engine. Watt's Teaching. Rankine's Curve 
Indices. 

CHAPTER III 

Action of Steam (Continued) \q 

Effect of Cylinder Metal. Leakage Theory. Re-evaporation 
Jackets. 

CHAPTER IV 

Steam: its Generation and Physical Properties 27 

Saturated Steam. Temperature-pressure Table. 

CHAPTER V 

Superheated Steam: its Properties 39 

General Resume of Superheating. Specific-volume Table. 

CHAPTER VI 

Steam-pipes and -valves 56 

Stresses. Expansion. Flow of Steam. Pipe Dimensions. 



X CONTENTS 

CHAPTER VII 

PAGE 

Superheat and Steam-turbines 62 

System of Regulation and Effect on Need for Superheat. 

CHAPTER VIII 

Behavior of Engines with Superheated Steam 66 

Valves. Limiting Temperatures. Action of Parts. Drop-valves 
and High Temperatures. Controlled Superheat. 

CHAPTER, IX 

Controllable Superheaters 71 

Methods of Control. Heat-inertia Effects. Mass-control. Water- 
control. Position of Superheaters. Air-drenching. Cast-iron 
Cores. McPhail's System. Cruse System. Foster's Gills. 

CHAPTER X 

Superheating as an Element in Steam Generation on the Stage 
Principle 86 

CHAPTER XI 

Superheaters 89 

Small Tubes. Location. Flooding. Materials. Cast Iron. 

CHAPTER XII 

Feed-water Heating 95 

Economy. A Stage in Steam Generation. Economy due to 
Fully Heated Feed. How Water-control Fits in with Heating 
Feed. The Normand Effect. The Four Elements of Steam 
Generation. A Plea for Scientific Steam-raising. 

CHAPTER XIII 

Examples of Superheaters 103 

The Foster: Its Portable Form. Cruse's Accumulator Type. 
Internal Cores. The Water-controlled Type. Diathermancy of 
Dry Steam. Action of Saturated Steam in Water-inspirator. 
Forms of Superheater Pipes, Water Cores, and Cast-iron Cores. 
The Ferguson Superheater. Its Drainage System. Its Location. 



CONTENTS XI 

CHAPTER XIV 

PAGE 

Independently Fired Superheaters 119 

Air Excess. The Need for Careful Furnace Design. 

CHAPTER XV 

The Practical Economy of Superheat 122 

Data from a Textile Factory. The Need for Sound Furnace 
Practice. 

CHAPTER XVI 

Superheat in Locomotives 134 

The Difficulties of the Locomotive Boiler: How Overcome. 
Haughton's Superheater. Canadian Experiences. 

CHAPTER XVII 

High Superheat . '. 139 

Velocity of Steam through Superheater. Schmidt's System. 
Throttling in Small Tubes. Time an Element in Superheating. 
German Experience. 

CHAPTER XVIII 
General Review 142 

CHAPTER XIX 
Useful Hints and Definitions, Tables, etc 147 



LIST OF ILLUSTRATIONS 



figure PAGE 

1 . Elementary Cylinder and Piston 10 

2. Development of Engine 10 

3. ' ' Double-acting Engine 10 

4. The Indicator-diagram 12 

5. Temperature and Pressure of Saturated Steam 36 

6. Variation of Temperature with Velocity . .'.- 60 

7. Variation in Heat Transfer with Velocity 60 

8. Steam-pipe Area per 1 ,000 Pounds per Hour 61 

9. Loss of Temperature with Pipe Sizes and Velocity Changes 61 

10. Diagram of Mean Temperature of Superheat 70 

11. " " Effect of Mass and Water-control 76 

12. Cruse's Accumulator Superheater 77 

13. " 32-pipe Water-controlled Superheater 78 

14. ' ' Independently Fired Superheater 79 

15. " « 32-pipe Superheater for Lancashire Boiler and Return-tube 
Boiler 80 

16. Header for Water-controlled Superheater 81 

17 a, 6, c. Foster's Gilled-^ipe Protection and Control 82-84 

18. " " Superheater in Edgemore Boiler 104 

19. ' ' Self-contained Superheater 105 

20. Superheater Tube without Internal Water-control Pipe 108 

21. " " with " " " 109 

22. Types of Iron and Water Cores for Controlled Superheaters 114 

23. Ferguson's Superheater 116 

24. Details of Ferguson's Superheater 117 

25. Independently Fired Water-controlled Admiralty Straight-tube 

Superheater 120 

26. Haugh ton's Shielded Locomotive Tube for Heat Conservation 135 

27. " Locomotive Superheater 136 

xiii 



TABLES 



TABLE PAGE 

I. Specific Heat of Various Substances , . . . 8 

II. " " "Gases • 8 

III. Temperature and Pressure of Saturated Steam 37 

IV. Schmidt's Values for Specific Volume of Superheated Steam based 

on Hirn's Experiments 55 

V. Loss of Temperature per 100 Feet of Pipe 59 

VI. Estimate of Steam Consumed per H.P. Hour by Engines using 

Superheated Steam 130 

VII. Properties of Saturated Steam 150 

VIII. Percentage of Saving Due to Feed-heating 151 

IX. Factors of Evaporation 152 

X. Pressure and Temperature of Low-pressure Steam 153 

XI. Some Conversion Factors 154 

XII. Steam-carrying Capacity of Extra-heavy Steam-pipes 155 

xv 



SUPERHEAT AND SUPERHEATERS 



CHAPTER I 
THE PAST HISTORY OF SUPERHEATING 

The first British patent for a superheater dates back to a 
period coequal with the birth of the United States as a separate 
and independent nation. The superheater then patented was 
similar in type to the many small-tube superheaters of the 
present day. But there does not appear to be any serious 
evidence of the use of superheated steam at that time. This 
first patent was granted to one Joseph Hateley in 1786. It is 
believed he had been experimenting for some years prior to 
this date. 

The early Cornish engineers, however, appear to have been 
closely on the track and to have at least realized the evils 
which, to-day, the modern steam-engineer recognizes as those 
which superheat is capable of amending. 

It cannot be believed for a moment that they were not 
fully cognizant of the existence in the cylinder of a steam- 
engine of large volumes of water where only steam was wanted. 
James Watt in his invention of the separate condenser recog- 
nized that when cold water was admitted to the working 
cylinder, that cylinder was rendered cold and very much steam 



2 SUPERHEAT AND SUPERHEATERS 

was found to be necessary in order to supply the cylinder with 
steam so that the falling pump-rods might lift the piston 
against the pressure of the atmosphere. This it could only 
do of course when there was steam below the piston at atmos- 
pheric pressure. The first steam admitted to the cold chilled 
cylinder was destroyed in overcoming the chill of the last 
douche of cold condensing water. Then when the cylinder 
was reheated to 212° F. any further steam could live as steam. 
In order to get over this very serious difficulty, Watt provided 
a separate condenser into which he injected cold water to 
condense the steam exhausted from the working cylinder. 
Cold water did not now touch the interior walls of the cylinder 
and the condensation of steam subsequently entering the 
cylinder was very much reduced. This of course, though not 
commonly recognized as such, was the first step in the com- 
pound working of steam which has since grown to such extensive 
proportions in the triple- and quadruple-expansion engine. 

The Cornish engineers, ably led by Captain Richard Trevi- 
thick and others, devised a system of steam-working which 
was another very considerable advance in the right direction. 
Prior to this, the steam from the boiler was drawn into the lower 
end of the cylinder of a beam engine by the rising piston. This 
was drawn up by the weight of the descending spear-rods of the 
great pumps. These rods did the work of pumping water as 
they descended. They were lifted by the pressure of the 
atmosphere above the piston at the time when there was a 
vacuum below the piston caused by condensation of the steam. 
Now the next improvement to follow the separate condenser 
was to close the top of the cylinder, which had hitherto been 
open to the atmosphere. The piston-rod was made to pass 
through an air-tight stuffing-box or gland. The descending 
piston was now forced down, not by direct air, but by steam, 
at or above atmospheric pressure, which entered the top of the 



THE PAST HISTORY OF SUPERHEATING 3 

cylinder from the boiler. Steam-pipes had begun to be employed 
where hitherto there had been little more than a short neck 
connection from the boiler. 

When the downward stroke of the piston was complete and 
the time had arrived for it to be drawn up again by the spear- 
rods, the bottom of the cylinder was closed off from the con- 
denser and a communication was opened between the top 
and bottom ends of the cylinder. The steam which filled the 
cylinder above the piston was now transferred below the piston 
by the upward movement of this as the spear-rods descended. 
There was equality of pressure on the two faces of the piston: 
the piston having arrived at the top of the cylinder, the trans- 
fer-passage was closed, the bottom of- the cylinder was opened 
to the condenser, and the top of the cylinder was opened to 
the boiler; the piston descended as before into the vacuum 
formed below it. But little thought is required to show that the 
working end of the cylinder was never exposed to steam at a 
temperature very much below 212° F. True, some pressure 
was lost during transfer round the piston, because the lower 
end of the cylinder had been in communication with the con- 
denser and was cold. But the system of working tended to 
keep water out of the cylinder bottom. Any mass of water 
lodged probably in the lower port and was at once discharged 
unevaporated to the condenser, and the top of the cylinder 
was a stage removed from the cold condenser. This was a 
most important improvement, and we cannot doubt that the 
Cornish engineers who so deliberately worked out this system 
must have been keenly alive to the benefit, to use Watt's words, 
"of keeping the cylinder as hot as the steam that entered it." 
And so we find fire-flues carried round the cylinder to keep it 
warm, and even a fire built under the cylinder. This must 
have been the cylinder of a Bull engine, that type of pumping- 
engine devised by Captain Bull in which, as would naturally 



4 SUPERHEAT AND SUPERHEATERS 

happen after the introduction of the stuffing-box and closed 
cylinder, the piston-rod came out below the cylinder, or per- 
haps the pump-rods were carried by a great cross-head from the 
piston-rod and above the cylinder, so that steam was admitted 
below. 

But the point need not be labored. We can gather from 
early records sufficient to show that the presence of water in 
the working cylinder had been recognized and its removal and 
prevention attempted. It is, however, not so clear that the 
production of water in the cylinder was so clearly understood 
in after-years when men had forgotten the early days of directly 
water-cooled cylinders; for we seem to arrive at a period of 
less striving, as though, with the use of higher-pressure steam 
of 15 and 20 pounds gage-pressure, the old influences did not 
operate or were thought not to do. 

Attracted by the results of his investigations into the 
behavior of locomotives, D. K. Clark put forward afresh and 
originally the modern theory of cylinder condensation. He 
pointed out that the mere fact of the range of pressure in a 
working cylinder being from that of the boiler clown to that 
of the atmosphere, implied a corresponding range of tempera- 
ture of the steam with which the cylinder was in contact; and 
he seems to have been the first of the modern steam-engineers 
to realize how very serious was the effect of this range of tem- 
perature in the working cylinder. Hirn, the famous Alsatian 
engineer, thoroughly investigated the subject by tests of actual 
engines, and may be said to have established the theory 
of the action of the cylinder-walls as now understood by all 
intelligent steam-engineers. It is true that great efforts have 
been made of late years to establish a new theory We may 
term this new school the " Leakage School," because they have 
endeavored to show that most of the effects hitherto attributed 
to condensation are due to leakage past the piston and valves. 



THE PAST HISTORY OF SUPERHEATING 5 

Unfortunately for their theory, the figures they advance to 
prove their case can be fully as easily reasoned out to support 
the established theory. It is also easy, when endeavoring to 
found a new theory, to arrange the facts so that what is sought 
shall be found, and it cannot be said that the new school have 
been so discriminating in their choice of engines on which to 
experiment as to carry conviction to the unbiassed mind. If 
leakage is to be proved, it can be best proved by experimenting 
on some small unsatisfactory engine that is most likely to 
leak. The present is not, however, the opportunity for a full 
discussion of the rival theories. It is the occasion for dis- 
cussing means for diminishing the bad effects which result 
from cylinder condensation, and without further animadversion 
on the errors of the new theory, it will be at once assumed, as 
the basis of this book and the arguments it contains, that the 
old-established theory is the more correct — an assumption 
that is borne out by the fact that the remedy proposed meets 
the evils which it is intended shall be met. 

This granted, the subject of superheat and superheating 
will now be discussed on the basis of the need for superheat, 
the causes which, in the practical working of the steam-engine, 
render superheat necessary, and the means and apparatus to 
be adopted to obtain superheat. Before discussing superheated 
steam and its properties, it will be assumed provisionally 
that superheated steam is a vehicle to convey heat to some 
place where it is required for special use or any other purpose. 
With this understanding the reasons for the need of this heat 
will be first discussed. Naturally these will divide themselves 
into two main divisions: first, the use of the substance steam 
in the steam-engine; and secondly, its use in other ways in 
the arts and manufactures. It will be necessary to go some- 
what outside the narrow treatment of the subject, at least 
as regards the steam-engine. 



6 SUPERHEAT AND SUPERHEATERS 

The avoidable losses in the steam-engine have been at- 
tempted to be cured by means other than superheat — the 
jacket variously arranged, for example with steam, liquid, 
or gaseous filling; and it will be necessary to refer to these 
in order to help to define the behavior of superheat and to 
render the subject more clear by taking a whole view of the 
various phenomena attached thereto, and the lines on which 
men have worked in endeavoring to arrive at a solution of the 
problem. 



CHAPTER II 

SPECIFIC HEAT OF MATERIALS 

All substances when in contact with one another at different 
temperatures tend towards the same temperature and ulti- 
mately acquire it. Where two substances are in contact, one 
of them will lose heat, the other will acquire heat. If the two 
substances have the same mass or weight, it might at first be 
surmised that the final temperature would be just half-way 
between the two extreme temperatures. This, however, is 
not found to be the case, because when one substance changes 
its temperature by ten degrees it may lose only as much heat 
as would be lost by the same weight of the other substance 
changing through one degree of temperature only. The heat 
lost or gained by unit weight of any substance when its tempera- 
ture changes one degree is known as the specific heat of that 
substance, and the values of the coefficients of specific heat are 
very different. Chemically considered, substances have very 
similar specific heats per atom or molecule. Dulong and 
Petit, who determined the specific heat of many substances, 
observed that the specific heat decreases as the atomic weight 
increases, and that the product of the atomic weight W into 
the specific heat H was a nearly constant quantity. The atom 
is the unit of thermal changes. All atoms are equithermal and 
the product of W and H is the almost constant quantity 6. 
For purposes of this book the values of the specific heat of but 

7 



8 SUPERHEAT AND SUPERHEATERS 

few substances will be required. For cast iron the value is 
usually taken sufficiently correctly as 0*1 11 or \ that of water, 
which has the datum value of 1*000. The British Thermal 
Unit is the amount of heat that is necessary to raise the tempera- 
ture of one pound of water from 39° to 40° F. Practically 
it is the amount necessary to change the temperature 1° F. at 
any initial temperature, but as a close scientific fact the specific 
heat varies slightly with variation of temperature. Water has 
the greatest specific heat of any known substance excepting 
the gas hydrogen. 

In Table I will be found the specific heat coefficients of a 
few of the materials of engineering, and in Table II those of a 
few of the gases most met with in steam-engineering. These 
values are necessary in many calculations. 

Table I 
SPECIFIC HEAT OF VARIOUS SUBSTANCES 



Aluminium 


= 2143 


Ice 


= 5040 


Brass 


= 0-0940 


Lead 


- 0-0314 


Cast iron 


= 0-1216 


Mercury- 


= 0-0333 


Average Coal 


= 02412 


Mild steel 


= 01158 


Copper 


= 0965 


Water 


= 10000 


Fire-brick 


= 2000 


Wrought iron 


= 0-1146 


Gun-metal 


= 0952 








Table 


II 





SPECIFIC HEAT OF GASES 



Constant Pressure. Constant Volume. 





2375 


Carbon dioxide 


02160 


Carbonic oxide 


02450 

3 4100 


Oxygen 


02170 




■ 2440 




0-4790 


Steam at 2000° 


11 " 4000° 



1710 
1730 
4146 
1548 
1730 
3700 
9000 
3200 



SPECIFIC HEAT OF MATERIALS 



The Action of Steam in the Cylinder 

It may now be well to investigate the working of the steam- 
engine and that of the fluid by which it is actuated. The 
steam-engine is a heat-engine: heat is the form of energy by 
which it is moved, just as gravity is the form of energy which 
revolves the water-wheel. The water does not do the work 
of turning the wheel, but it acts as the vehicle or agent through 
which gravity is enabled to exert its influence. The coupling- 
chain of a railroad-car does not haul the train. It acts merely 
as the vehicle by which the pull of the locomotive is enabled 
to act upon following vehicles. Just so with steam. Steam is 
but the vehicle of heat. It is as well to impress this great 
fact upon the mind, because there is a tendency to regard 
steam on the basis of its volume, irrespective of its contents 
of heat. By reason of this fallacious method of regarding 
steam, erroneous ideas have manifested themselves in regard 
to superheat upon which arguments have been founded that 
do not agree with correct principles as regards heat, as will 
be seen later. 

A steam-engine is set in motion by the action of steam 
which presses upon the whole interior surface of the working 
cylinder. The working cylinder of a steam-engine is a vessel 
the capacity of which can be varied. The easiest form of vessel 
the capacity of which can be made to vary is the cylinder with 
one loose end. The loose end is made in the form of a circular 
disc, and it is a close fit inside the cylinder. The loose disc 
is called a piston. If it be pushed up towards the closed end of 
the cylinder, Fig. 1, and steam be admitted from a boiler at a 
greater pressure than that of the atmosphere, the piston will 
be driven towards the other end of the cylinder. If to the 
piston there be hinged a rod the other end of which is hinged 



10 



SUPERHEAT AND SUPERHEATERS 



on the pin of a crank, the single-acting engine of Fig. 2 will 
have been evolved. In the double-acting engine, Fig. 3, the 
cylinder is really made up of two such variable-volume vessels 
placed back to back, and the same movable boundary or piston 
serves for both ends. One end of the double vessel increases in 
volume, while the other end diminishes. A straight-moving 
rod is attached to the piston and serves to carry the hinged 




Fig. 1 



Fig. 2 



end of the connecting-rod entirely outside the cylinder. Thus 
the ordinary double-acting engine is evolved and, as in the 
single-acting engine of Fig. 2, the variation of volume of the 
cylinder enables the steam to do work on the piston, and this 
work is made to push the crank-pin or otherwise do work against 
some resistance. Thus developed the common steam-engine 
in its crude form. With all this, however, readers are familiar. 




Fig. 3 

When steam is admitted into the cylinder it enters between 
the end of the cylinder and the face of the piston, and it pushes 
upon the piston, and this moves and does work. Unless steam 
can increase its volume it cannot do work. So long as the 
communication is open between the cylinder and the boiler 
the pressure in the two vessels will remain the same. Con- 
sequently the steam in the cylinder does no work, for it does 



SPECIFIC HEAT OF MATERIALS II 

not change its volume. The work of pushing the resisting piston 
must come direct from the furnace, for we may assume for the 
purpose of the present argument that steam is produced in 
the boiler at the same average rate that the engine is consum- 
ing it. The steam is not changing its volume, but the water 
next the furnace-plate is changing into steam and this pushes 
in front of it the previously formed steam in the steam-space, 
the pipes and cylinder, and so moves the piston. This method 
of using steam is very wasteful, however. Steam enters the 
cylinder throughout the full length of the stroke or travel of 
the piston, and having pushed the piston to the end of the 
cylinder, an exhaust-valve is opened and the steam escapes 
to the condenser or to the atmosphere at full boiler-pressure, 
no work having been got out of it since it changed on the 
furnace-plate from the liquid condition and pushed older 
steam before it. 

Watt realized the wastefulness of this method of working 
and so changed the movement of the valve of admission that 
no steam was admitted to the cylinder during the later portions 
of the piston movement. One volume of steam was admitted 
and the admission closed while the capacity of the cylinder 
continued to grow, as the piston moved, until its volume was 
two, three, or more times what it was when the admission was 
closed. 

In Fig. 4, which is the well-known steam indicator-diagram, 
the rectangle AC represents the work done by the furnace 
upon the piston during the period of admission of steam to the 
cylinder. When the admission-valve is closed, the piston, con- 
tinuing to move, enlarges the capacity of the cylinder and the 
imprisoned steam falls in pressure almost inversely as it increases 
its volume. Each addition of the initial volume, represented 
by the rectangle AC, which also represents an amount of work 
AC, will now represent so much additional work. In travelling 



12 



SUPERHEAT AND SUPERHEATERS 



from X to C work = ABCX was done on the piston by the direct 
furnace-heat. Then when the steam expanded behind the 
moving piston it did work by virtue of its own expansion to 
the double volume XB, to the extent of the area of the irregular 
figure BD. A further amount of work = EF was done when 
the third volume, DF, was generated, and so on volume by 
volume of increased space occupied enabled the imprisoned 
steam to do more work, until in expanding to eight times the 
initial volume the expansion generates the work OP and the 




Fig. 4 

initial volume XC becomes the volume XR. The increased 
work clone by the expanding steam is represented by the area 
CBNP, and this area will be found to be fully double the area 
ABCX, showing that the steam has done over three times as 
much work in all as it would do if not expanded. Calling the 
initial area 1, that of the expansion area, if carefully measured, 
will prove to be 2*0794, so that the total area is 3' 0794. 

The diagram has been drawn according to Charles' law of 
gases, which states that, at the same temperature, the pressure 
of a gas is inversely as the volume occupied. Thus at any 
point along the curve BN the product of the horizontal distance 
to AX and of the vertical distance to XP will be a constant 






SPECIFIC HEAT OF MATERIALS 13 

figure. Expressed mathematically this gives p. v — const., or 
pressure x volume = constant. Now this is the equation to the 
hyperbola, and the curve BN is a rectangular hyperbola, and 
the rectangular hyperbola is the curve of isothermal expansion 
of a perfect gas, or of a perfect gas, such as air, expanding at 
one constant temperature, which is only possible where heat 
is added during expansion; for when a gas expands against 
a resistance it does work and is cooled thereby. 

Steam, however, is not a perfect gas. As it expands in 
the cylinder of an engine it does work and cools, and the curve 
of expansion of saturated steam is not the hyperbola, but is 
a curve which falls more and more below the hyperbola as 
expansion proceeds. The mean forward pressure of a gas 
expanding isothermally may be represented by the following 
formula: 



p / l + hyp.log.r 



)• 



where p = the mean pressure, 

P = the initial pressure, or AX of Fig. 4, and 
hyp. log. r = the hyperbolic logarithm of the number of ex- 

XP 

pansions, or the ratio — -. 

This is a very useful formula in practice, because it happens 
to coincide fairly with the results of practice so far as the 
actual indicator-diagram shows to be the case. While the 
expansion-curve of saturated steam falls somewhat below the 
hyperbola, the curve of the actual indicator-diagram in a good 
tight engine always rises above the hyperbola near the toe of 
the diagram, but falls below it over the length BE in the early 
stages of expansion, and the effect is to cause the mean pressure 
as shown by the indicator-diagram to differ very little from 
that shown by the true hyperbola. This is, however, a mere 



14 SUPERHEAT AND SUPERHEATERS 

coincidence, for saturated steam is not a perfect gas, it does 
not expand isothermally, and it does not trace even an adiabatic 
curve of expansion or the curve of expansion followed by a 
gas when experiencing neither gain nor loss of heat. 

According to Rankine the curve of adiabatic expansion 
of saturated steam is given by the formula PV'^ = constant, 
which shows that the pressure falls more quickly than the 
volume increases. For saturated steam maintained dry he 
gives the formula PV J * = constant. These formulas require 
a table of logarithms to work them out. 

For superheated steam Rankine gives the formula PV 1 ' 3 = 
constant, while for air the equation becomes PV r * 08 = 
constant. In practical steam-engineering these curves do 
not enter much into our work, but it is desirable to know of 
them and to recognize that the index to the curve of super- 
heated steam, to wit, the figure 1*3, tells us that the expansion- 
curve must fall very rapidly and that the area below such a 
curve is therefore much less than the hyperbolic area. If this 
detail be worked out, it will probably suffice to teach the 
student that there is nothing in the argument that superheated 
steam makes any gain by virtue of its greater volume as com- 
pared with saturated steam. It is essential to insist on this 
point, because otherwise we should be allowing an argument 
contrary to fact, that a heat-engine works not by virtue of 
heat but by virtue of the vehicle of the heat. 

So many factors go to the production of the actual indi- 
cator-diagram that it is not possible to give any mathematical 
expression that will set forth the properties of the actual curve 
of expansion. The curve of the indicator-diagram is in fact 
primarily a curve of expansion, but modified by condensation 
and by evaporation. The cylinder is at once both a condenser 
and a boiler at one and the same time over different portions 
of its internal area. That is to say, steam is condensing upon 



SPECIFIC HEAT OF MATERIALS 15 

freshly exposed cold surfaces as the piston moves, and water 
is re-evaporating upon surfaces already made hot, and water 
re-evaporates also because it has been produced from initial 
steam and is, now that the pressure has fallen, hotter than 
the steam at this pressure. Some of this hot water will re- 
evaporate and help to fill up the lower end of the expansion- 
curve. 

The specific heat of superheated steam is about 0*48 at 
constant pressure when heated away from water with the 
boiler-pressure behind it and with freedom to expand. 

At constant volume, i.e., bottled up in a closed vessel, the 
specific heat is 0*346, but here the pressure would rise and 
these figures are generally accepted as correct and much em- 
ployed as the basis of calculations; but they are after all only 
approximate, since the specific heat of superheated steam rises 
as the temperature rises and the condition of a perfect gas 
is attained. In modern practice and pressures the specific 
heat may be taken at 0*60. 

To return to the superheater in open communication with 
the steam-space through a pipe; it is known that to generate 
dry saturated steam at 100 pounds pressure absolute demands 
1181-9 B.T.U. from water at 32° F., the temperature of the 
steam being 327*7° F. and its volume 4*34 cubic feet per pound. 

To superheat this steam by 200° F. to 528° F. and to a 
volume of 5*43 cubic feet per pound requires 0*48 B.T.U. per 
pound (if not more), or 200X0*48 = 96*1 B.T.U. additional heat. 
Thus 1181*9 + 96*1 = 1278 B.T.U., being an increase of 8*13 
per cent, only, in the total heat, and this represents the nominal 
increase of efficiency and power in the steam due to super- 
heating as calculated on the B.T.U. at the outlet of the super- 
heater. But the actual increase of effect is much more than 
these figures would imply. 



CHAPTER III 

THE ACTION OF STEAM IN A CYLINDER. THE EFFECT OF 
CYLINDER METAL 

The actual behavior of steam in the cylinder of an engine 
may now be traced out. 

Let a beginning be made from an engine at rest and cold. Let 
it be supplied with steam at 361° F. temperature or 155 pounds 
absolute pressure. It is usual to warm up the cylinder before 
starting the engine to run. This warming up never goes beyond 
212° F., for it is always carried out with the cylinder drain- 
cocks open and therefore at atmospheric pressure. The cylinder 
when cold will have a temperature of about 62° F., so that to 
heat it up to 212° will require an additional 150° of temperature. 
Since the specific heat of iron is 0*11, the total heat-units 
absorbed per pound of the cylinder metal will be 150X0*11 or, 
say, 17 B.T.U. In one pound of steam there are approximately 
966 B.T.U. of heat above 212° F., and there are 1116 B.T.U. 
above the initial temperature of 62° F., at which warming up 
is commenced. The mean of these two quantities is 1041 
B.T.U., so that one pound of steam will serve to heat up 61 
pounds of cylinder metal to 212° F. The cylinder has yet to 
acquire the temperature of the boiler, or 361°, so that a further 
temperature rise of 361-212 = 149° must be gained from the 
steam, which will lose about 1000 B.T.U. per pound, and each 
pound will therefore heat up about 60 pounds of cylinder 

16 



THE ACTION OF STEAM IN A CYLINDER 17 

metal. Stated briefly, a modern engine will be heated up to 
boiler temperature by the destruction of steam to the weight 
of one thirtieth of that of the cylinder or, say, 3 per cent. 
Now when fully set to work the steam enters the cylinder at 
361° and continues at this temperature for a time. The 
induction-valve is then closed and the steam begins to expand 
until it has fallen, say, to 200° F., the pressure being then 
somewhat below the atmosphere. The exhaust-valve is then 
opened and pressure falls to that of the condenser. Let this 
be equivalent to 100° F. of temperature. The steam commenced 
at 361° F. and has dropped to 100° F. It has passed through 
a range of temperature of 261° F. Now it is impossible for 
steam or any other substance to be in contact with iron or 
other substance without the substances in contact with each 
other endeavoring to attain the same temperature. 

Since the temperature of the steam is varying, that of the 
cylinder must also vary all the time. It will strive to attain 
361° F. and it will strive to fall to 100° F. Its mean tempera- 
ture will be somewhere between the extreme limits. Grant- 
ing this, let it be assumed that steam enters a cylinder the 
temperature of which is 341°, whilst the steam is at 361° F. 
The steam finds itself between the cylinder-head and the 
piston-face, and it is also exposed to a small portion of the 
cylinder-body. Saturated steam is in a critical state. It 
parts with heat rapidly and condenses, and it does this now that 
it is between the above two cold areas. Each pound of steam 
can lose 859*7 heat-units in becoming water. This amount is 
the latent heat of steam at 361° F., as found in any steam- 
table in most pocket-books. The condensation of this steam 
does not cause any reduction of pressure in the cylinder, for 
this is in free communication with the boiler, and as fast as 
the steam condenses its place is taken by fresh supplies from 
the boiler. If the piston moves slowly, the whole interior of 



18 SUPERHEAT AND SUPERHEATERS 

the cylinder becomes as hot as the steam. It cannot be ac- 
curately told to what depth the inner wall of the cylinder is 
penetrated by heat, nor how deeply the fluctuation extends 
into the metal. Doubtless some little way below the surface 
the cylinder metal will be at a steady average temperature, 
the fluctuations of the inner skin dying down to nothing. As 
a matter of interest and to serve the purpose of an illustration, 
it is customary to assume that the inner skin of the cylinder 
to the depth of one one-hundredth of an inch — O'Ol" — follows 
the fluctuation of the steam temperature. Knowing the size 
of the cylinder, its inner area at the time of cut-off may be 
easily calculated. Assume a cylinder 20 inches diameter, 
with the cylinder-body uncovered by the piston for a length of 
6 inches at the cut-off point. The piston-rod is 3 inches 
diameter. How much inner surface is exposed to steam? 
The surfaces measure up as follows : 

Cylinder-head 20 2 X'7854 =314 square inches 

Piston-face 20 2 X*7854 =314 " 

Cylinder-body 20 X 3' 1416 X 6 = 377 " 

Piston-rod 3 X3" 1416x6= 56 " " 

Ports, etc., say 100 " " 

Total area 1161 " " 

This represents 11' 6 cubic inches of metal if the surface or 
skin be supposed affected to a mean depth of 0*01". Let it 
be called 3 pounds by weight. Since the specific heat of the 
metal is 0*11, how much heat will it absorb in rising from 
101° F. to 361° F. or 260° F.? It will absorb 3xO'llX260 = 
86 B.T.U. nearly. The weight of steam that will supply this 
heat will be 0'10 of a pound, for the latent heat of steam of 
361° F. temperature is 859*7 B.T.U., and this divided into 86 
gives 0*10 as the weight of steam condensed. 



THE ACTION OF STEAM IN A CYLINDER 19 

The volume of steam present at the point of cut-off will be 
approximately 20 2 X0'7854X7=2199 cubic inches, weighing 
0*443 pound. On the above assumption, therefore, the steam 

present in the cylinder is only ^ or 81 per cent, of the weight 

which entered it. The remainder is present as water and 
amounts to 19 per cent of the total steam which entered the 
cylinder. This percentage varies in practice from 20 to 50 
per cent of the initial steam entering up to the point of cut-off. 
The figure 7 in the last equation is used in place of 6 — the 
length of cylinder exposed at cut-off — in order to allow for the 
volume of ports, etc. It is merely an assumption approxi- 
mately correct. The figure 0*543 =0*443 + 0*10, as found above 
for the weight condensed. 

We have now arrived at the point where the piston is at 
the point of cut-off. The cylinder and piston surfaces have 
acquired the temperature of the boiler-steam. Expansion is 
ready to commence and there is present in the cylinder 0'10 
pound of water, also at 361° F. of temperature and wanting 
only the addition of latent heat to reconvert it into steam. 

As the piston moves farther along the cylinder it uncovers 
more cold surface, and this freshly exposed surface will condense 
more steam. At the same time, however, the steam is falling 
in pressure and the water which is at 361° F. finds itself hotter 
than the temperature proper to the pressure and some of the 
water flashes into steam. The cylinder and piston are now 
hotter than the partly expanded steam and begin to part with 
heat to the dew or moisture which has collected upon their 
surfaces. There are thus two conflicting influences at work. 
There is the cooling influence of freshly uncovered cylinder 
surface causing condensation, and there is the evaporation of 
hot water. At first the condensation prevails and the expan- 
sion-curve falls below any proper curve of expansion. Then as 



20 SUPERHEAT AND SUPERHEATERS 

pressure drops re-evaporation becomes greater; the cylinder- 
wall temperature is no longer able to condense the now colder 
expanded steam. Nothing now proceeds but evaporation; 
the curve of the indicator-diagram becomes parallel with and 
finally flatter than any true curve of expansion. The evapora- 
tion becomes more and more active and many indicator- 
diagrams may be found, especially from slowly running engines, 
in which the curve actually begins to slope upwards near the 
toe of the diagram. Calculating the cylinder volume at any 
given point before the exhaust-valve opens, it will be found 
that the pressure of steam now present in the cylinder is such 
that the weight of steam must now be very much greater than 
could be measured at the point of cut-off. This increased 
weight of steam now measurable was present as water at the 
point of cut-off. 

It might here . not be out of place to refer again to the 
new theory that these changes in volume are due to leakage, 
past the valve, of fresh steam. That this is not so may be 
taken as fairly well proved by the fact that, immediately and 
for some time after passing the point of cut-off, the expansion- 
curve rapidly falls at a rate far beyond any expansion slope. 
Then finally it is found to be falling at a rate very much less 
than the expansion rate. Are we to take it that at the time 
of cut-off the steam is slipping away past the leaking piston; 
that the piston then becomes steam-tight, and that fresh steam 
now begins to leak into the cylinder? 

If actual indicator-diagrams be attempted to be considered 
by the light of leakage, it will be found requisite to demand 
alternate leakage and non-leakage past pistons and valves 
of quite a complicated order, if the leakage idea is to be main- 
tained. The author has not found that the steep curve of the 
indicator-diagram which follows the point of cut-off has been 
attempted to be explained by the school of leakage theorists, 



THE ACTION OF STEAM IN A CYLINDER 21 

and unless they can explain the diagram thoroughly without 
irrational assumptions, their theory will scarcely hold good. 

The assumption that the thickness or depth of film of the 
inner wall of the cylinder which varies the full range of the 
steam-pressure is 0*01" is purely gratuitous. The depth to which 
the cylinder varies its temperature is not a fixed quantity. It 
varies with the period during which the cylinder surface is ex- 
posed to the steam, and it is quite obvious that it will be less 
with increase of rotative speed, because this governs the ex- 
posure time per stroke. It must also vary with the ratio of 
cut-off to stroke, especially since less exposure to boiler-steam 
necessarily means a greater exposure to the expanding steam 
and a relatively greater exposure to the exhaust. There are so 
many variables that only approximations can be made in any 
empirical formula designed to meet the case. The fact remains 
that in all engines the inner skin of the cylinder absorbs heat 
from the incoming steam and causes a serious fraction of this 
to condense. 

It is simply a question of time as to how nearly the cylinder 
attains to the boiler temperature. Possibly the end of the 
cylinder and the face of the piston approach somewhat nearly 
to the boiler temperature. But as soon as the steam admission 
ceases, the confined space, still increasing as the piston moves, 
enables the steam to expand. In expanding it falls to a lower 
temperature and is now in presence of water hotter than itself, 
as well as with the metal of the cylinder, also hotter than the 
steam. But this is not necessarily the case during the first 
few inches of expansion travel of the piston, for there is abun- 
dant evidence to be gathered from indicator-diagrams that 
condensation may proceed after expansion has begun, since 
there are cold cylinder-walls still being uncovered by the piston. 
The time comes, however, when the cylinder ceases to condense 
any further steam. The reason is twofold. In the first place, 



22 SUPERHEAT AND SUPERHEATERS 

all the internal surface of the cylinder up to cut-off has en- 
deavored to attain boiler-pressure and is therefore compara- 
tively hot. Secondly, the partially expanded steam is cool. 
There is also hot water in the cylinder formed during admis- 
sion, and this is now rapidly drying up. Re-evaporation 
gets more active as the piston approaches the limit of 
its travel, and if the amount of steam present in the cylin- 
der as steam be now measured from the indicator-diagram, 
it will be found considerably in excess of that measured 
at cut-off. The difference is the net re-evaporation. When 
the exhaust-valve opens to let out steam to the condenser, 
much of the remaining water, if any, and the engine is suitably 
designed, will be shot out to the condenser, but the water 
which is present as dew on the cylinder-walls will all be evap- 
orated, and the cooling effect of this evaporation is very great. 
Meantime the cylinder is always losing heat outwardly to the 
atmosphere, and all this heat must be supplied from within. 
It is checked by clothing the cylinder in hair felt or other non- 
conducting material, but heat must always be lost outwardly 
unless a jacket be present. Thus there is an ever-recurring 
cycle in which the cylinder is alternately hotter and colder 
than the steam within it. That this alternate cooling and 
heating must represent loss is obvious when it is considered 
that most of the steam condensed is taken from useful work 
early in the working stroke, and when it again becomes steam 
it is when it has little time left in which to work or when it 
actually retards the engine by raising the back-pressure. 

The action of the cylinder is to take out of the steam, or 
out of a part of it, its power of doing work and to return this 
power to it when it can be of no use but is rather a positive 
agent to check the engine. In order to cure this loss Watt 
perceived that it was necessary to maintain the cylinder as 
hot as the steam that entered it. To effect this end he encased 



THE EFFECT OF CYLINDER METAL 23 

it m a jacket. Now the jacket is a narrow space filled with 
steam at or near or above the boiler-pressure, and its primary 
effect is of course to supply all the losses from the cylinder 
which get away by external radiation to the atmosphere. The 
jacket sends heat out on both its boundaries, outside to the 
external atmosphere and inwards across the wall of the cylinder 
into the cylinder. Instead of heat flowing out from the interior 
of the cylinder, it now commences to flow inwards. It en- 
deavors to supply to the cylinder the heat that has been taken 
from it during expansion and exhaust. Let the course of events 
be assumed to aid us in realizing what the jacket may do. 
Starting at the initial admission, the steam finds that the 
cylinder is now hotter than it was, less steam is condensed, and 
when the exhaust is opened there is less water to re-evaporate 
and the cylinder loses less heat from that cause and is still 
hotter than it was during the first stroke after applying the 
jacket steam. Thus even less steam is condensed and the 
counter-effect of re-evaporation is again reduced. The effect of 
the jacket is thus seen to be cumulative. But generally speak- 
ing, the steam-jacket has been a disappointment. It has been 
badly designed and badly worked. It has become filled with 
air and very poor attention has been given it. It has been 
badly drained, has been choked with air, and has even been 
fed with exhaust-steam. Then the most important parts, the 
cylinder-cover and the piston, have been left unjacketed. 
Properly to work a jacket there should be a special jacketing 
boiler at a pressure somewhat higher than the main boilers, 
and the whole of the steam made by this boiler should be 
allowed to pass through the jackets and escape to the main 
boilers through a reducing-valve up to 5 or 10 pounds pressure 
difference. The jacket thus swept by the whole of the steam 
from one boiler will always be kept clear of air and water 
and the maximum efficiency will be secured. 



24 SUPERHEAT AND SUPERHEATERS 

Since this jacket is losing heat all the time, water is being 
formed within it, but this water is under constant pressure 
and does not re-evaporate. It passes away to the drains and 
cannot affect the temperature of the cylinder. It is true that 
the jacket may warm the exhaust-steam to some extent, but 
the point to be noted is that there is less water in the cylinder 
to be re-evaporated, that the cylinder is thereby less cooled 
by the evaporation effects, and that less steam must be initially 
condensed. Here the effect of preventing some water is to 
stop the production of other water, and if there is no water' to 
be re-evaporated there can be no cylinder coolirg. True, the 
exhaust-steam may abstract heat from the cylinder, but it is 
very far inferior to hot water as a cooling agent, for as it dries it 
becomes an exceedingly poor picker-up of heat from the walls 
which enclose it. The water formed in the jacket may go back 
to the boiler direct. 

Watt fully realized the action of steam-jackets, for he simply 
made them a thoroughfare whereby steam travelled on its way 
to the cylinder. This device is regarded by some with much 
disfavor, on the ground that the steam passing through the 
jacket is rendered wet, and they argue that it is unfit to be used. 
This view is, however, superficial. The anteroom jacket can be 
made to serve as an excellent separator and can be drained, 
and it is a good design. What really happens is that the steam 
is partially condensed in the jacket and not in the cylinder. 
Condensation is transferred from the cylinder to the jacket, 
and the condensed water is maintained at boiler-pressure. 
It does not re-evaporate and therefore does not exercise any 
cooling effect on the metal about it, such as the fluctuation of 
pressure within a cylinder enables it to do when it enters the 
cylinder. Needless to say, the jacket must be drained or water 
will get forward to the cylinder, there to act to the detriment 
of economy. The benefits of jackets have been greatly ques- 



THE EFFECT OF CYLINDER METAL 25 

tioned, but it has never been shown that with a good design the 
jacket did not act well. Of course it may easily be useless if 
badly applied or badly drained, and the author thinks that 
the only really reliable method is to pass through it all the 
steam from a subsidiary higher-pressure boiler. 

The principle of the jacket has been carried into effect, as 
already stated, by means of external flues around the cylinder, 
as in the old Cornish days. Glycerine jackets have been pro- 
posed, the glycerine being kept in circulation by a pump and 
heated in a coil of pipe external to the cylinder and in series 
with the jacket and pump. In all, the principle is the same— 
to check outflow of heat from a cylinder and produce a flow 
inwards. 

Needless to say, the effects of cylinder condensation will be 
made worse when the steam entering the cylinder is initially 
wet, for the wetness implies greater evaporation during exhaust 
periods and an initially cooler cylinder. 

A cylinder-wall being of some considerable thickness, the 
passage of heat from outside to inside of a cylinder will be slow. 
Similarly, as heat requires time to travel across a thickness of 
iron, the fluctuation of temperature of a cylinder-wall will 
not extend far inwards at full range. The range of fluctuation 
will diminish rapidly and soon a line will be reached where 
the temperature will remain unchanged. The net effect of 
the jacket will be to reduce the thickness of the inner surface 
film, in which all the temperature change has been supposed to 
occur, to some low r er figure. It was assumed, for illustration, 
as 0"01", and it may be assumed to be reduced to 0'009" or 
0'006" or some other smaller figure than the hundredth part 
of an inch assumed earlier. 

Seeing, however, .generally that the jacket applies heat 
w T here it is not wanted in order that some of it may reach to 
where it is wanted, it appears rational that an attempt should 



26 SUPERHEAT AND SUPERHEATERS 

be made to apply heat to the places where it is actually needed. 
Before further discussing the question of the action of heat 
in the cylinder it may not be amiss to discuss some of the 
physical properties of steam. 



CHAPTER IV 

STEAM. ITS GENERATION AND PHYSICAL PROPERTIES 

Steam is the vapor that is produced from water when this 
liquid is heated to such a point that the molecules of the water 
become endowed with an automobility sufficient to enable 
them to leave the water and enter the space above it. At all 
temperatures there would appear to be some vapor of water 
in the space above it, but the general. popular idea of steam is 
of that vapor which rises in bubbles through the water when 
heat is freely applied — when the water boils, in fact; a phenom- 
enon easier to realize than to explain in words. Steam formed 
in the presence of water is said to be saturated. By saturated 
many understand saturated with water. But heat saturated 
is rather to be understood. Saturated steam has a pressure 
or vapor tension that is peculiar to a certain temperature. 
One cannot vary without the other. If water is in a closed 
vessel and heat be applied, the water will begin to boil at a 
temperature of 212° F., because the vessel was closed up at 
atmospheric pressure. If an escape be opened so that the 
enclosed air may be driven off and the steam may escape as 
quickly as it is formed, the temperature will remain at 212° F. 
(at sea-level), and the pressure at 1 atmosphere absolute, or 
gage-pressure = pound; absolute pressure = 14' 7 pounds = 
1 atmosphere. If the outlet be now closed, the water will 
cease to boil at 212° F., for the accumulation of steam above 
the water will cause the pressure to increase and this will 

27 



28 SUPERHEAT AND SUPERHEATERS 

allow the water to become hotter, and for every pound of added 
pressure there will be so much increase in the temperature, 
the rate of increase of the temperature not being the same as 
the rate of increase of pressure. As the temperature rises the 
increase of pressure becomes more rapid. The steam remains 
saturated — with heat; that is to say, it contains practically 
the maximum amount of heat per cubic foot that it is 
possible to put into it. Such steam is not quite dry or 
perfect. Some few of its molecules are probably imperfect, 
and steam is perhaps not perfectly formed until it attains 
a temperature of 220° F., no water being present or other 
imperfect molecules would then rise into the dry steam. Any 
further heating beyond 220° F. or, at any other pressure, any 
addition beyond a few degrees above the temperature of forma- 
tion will destroy the right to the name of saturated. Such 
further temperature can only be acquired away from water, 
and steam so heated above the temperature of the water from 
which it is produced is termed superheated. Saturated steam 
contains heat in two forms, thermometric and latent. The 
thermometric heat of steam is the heat represented by the 
temperature of the water from which the steam is produced. 
At atmospheric pressure the thermometric heat in B.T.U. 
measured from 0° F. is 212 XF, where F is the coefficient of 
specific heat, which is 0*305 for saturated steam. 

It is usual, however, to measure the total heat from the 
temperature of melting ice, or 32° F. Then the formula for 
the total heat of evaporation of water is, when measured from 
32° F., 

# = 1146-6 + 0-305(*-212), 

where H = British thermal units, t = temperature of steam. 
For low ranges the formula is, when measured from 0° F., 

H = 1081-9 + 0- 305(^-32). 



STEAM. ITS GENERATION AND PHYSICAL PROPERTIES 29 

The British thermal unit is the amount of heat necessary 
to raise the temperature of 1 pound of water from 39° to 40° F., 
at which point water is at its maximum density. The differ- 
ence is not great if measured at 62° F., as is sometimes, but 
erroneously, done. 

The amount of heat in the water itself is 

5 = y-32) + 0-000011(^-32) 2 + 0-000000093(^-32) 3 , 
or approximately 

g=180-9+r02(*-212), 

where q = the B.T.U. in the liquid and t is the temperature. 
This formula holds good from 10 to 105 pounds absolute pressure 
or from 193° to 331° F. Obviously the latent heat of the steam 
must be the difference between its total heat and the heat in 
the water whence it was produced. 

Latent heat is that heat which when put into water at 
any temperature = x produces steam also at the temperature x. 
Since the heat is not apparent to the senses and does not affect 
the thermometer, it is said to be latent or hidden. In the 
kinetic theory of gases latent heat is that which when added 
to a whirling molecule of water gives to that molecule the 
power of automobility, or the § power to move from place to 
place. The latent heat of steam is given by the formula 

E = 965*7 -0'715(£ -212), 

where R = British thermal units. 

This formula also is good between 10 and 105 pounds 
absolute. 

The relation between the pressure and volume of dry 
saturated steam is expressed in the equation PS V065 = constant; 
that is to say, at all pressures the pressure multiplied by the 



30 SUPERHEAT AND SUPERHEATERS 

volume raised to the power T065 will be a constant quantity 
for the same unit mass of steam. 

It is obvious from what has been said of the critical rela- 
tions between the temperature and pressure of saturated steam 
that if anything be taken from the one, the other must also 
diminish, and vice versa. If saturated steam expands and 
does work, some of its heat is converted into work and some 
of the latent heat of the steam disappears, and the steam is 
so far partially condensed. 

If a vessel containing steam and water be compressed into 
smaller volume, some of the steam will be squeezed back into 
the water, because the pressure will be raised by the reduction 
of volume of the steam-space, and steam cannot exist at any 
pressure higher than that proper and critical to the water 
from which it is formed. 

The forcing back of the steam will raise the temperature of 
the water somewhat, because the latent heat of the now con- 
densed steam becomes apparent heat. Similarly if the capacity 
of the vessel had been enlarged further, steam would have 
risen from the water and would have obtained its latent heat 
at the expense of the thermometric heat of the water, and the 
pressure and temperature would fall. Nothing can be changed 
without other changes also occur. 

With so critical a fluid as saturated steam it is therefore 
easy to understand what has been said with reference to the 
action of steam in the cylinder of an engine, the interchange 
of heat between it and the cylinder metal, and the alternate 
and simultaneous production in the one cylinder of both steam 
and water and their similar simultaneous destruction at other 
zones in the same cylinder. 

We have seen, therefore, that the temperature, density, 
and pressure of saturated steam stand in a fixed relationship 
to each other. No one of them can vary without both the others 



STEAM. ITS GENERATION AND PHYSICAL PROPERTIES 31 

varying also. Change of temperature changes the amount of 
water in the vessel, the water increasing as the temperature 
falls and vice versa. 

In the steam-pipe heat is lost, but there is no pressure 
variation because the boiler keeps up a supply of steam at 
full pressure and temperature, and the heat lost is only the 
latent heat : w T ater is formed in the pipe and at boiler tempera- 
tures, and the steam is permeated with water. 

All tables of the properties of saturated steam refer to the 
dry article fully evaporated and carrying no suspended water. 

Best practice teaches that all steam-boilers working under 
ordinary conditions deliver wet steam carrying more or less 
unevaporated water either in fine cloud or actually in a mass — 
as when a boiler primes, as it will do when fed with certain 
impure waters or is too heavily forced. ..... 

Priming may be due to six causes in ordinary usage: 

1. To excessive evaporation or " forcing." 

2. To restricted water area and too small a steam-space 

for the amount of flow of steam. 

3. To defective circulation. 

4. To sudden or abnormal drafts of steam being taken 

from the boiler. 

5. To dirty feed-water. 

6. To unheated feed-water. 

Many of the great evaporative records so widely published 
are due to the reckoning of priming as true evaporation. It 
should be clearly understood that while the calorimeter in- 
strument for testing steam-dryness may give accurate measure- 
ments of the sample of steam supplied to it, no man has yet 
been able to take a sample of steam from a pipe which can 
be known to be truly or approximately representative of the 
quality of steam which is flowing in that pipe. Absolutely no 
boiler-test figures are worth the paper they are written upon 



32 SUPERHEAT AND SUPERHEATERS 

unless the steam has been supplied to show by superheat a 
temperature at least slightly above that of the boiler. Then 
only can it be known that the steam is not wet. 

The steam from the Lancashire boiler with its large water 
area is perhaps as dry as any steam ordinarily produced, but 
still it is not dry. 

Steam parts with heat more readily when wet than when 
dry, and the reduction to water is very great in long pipe 
ranges. Where live steam is used direct in vats and boiling- 
pans, the presence of so much moisture in the steam is very 
annoying, for it adds very little heat to the boiling liquor in 
the vat and it dilutes such liquor and may damage the dyes. 
Thus, to take an example to compare dry with wet steam, let 
one pound of dry saturated steam be assumed at 100 pounds 
pressure absolute, generated from water at 32° F. Then 

B.T.U. 
The total heat in this 1 pound of steam is 1181*90 



The total heat in 1 pound of wet steam and water con- 
taining 30 per cent of water by weight and reduced 
20° in temperature and 25 pounds in pressure is as 
follows : 

(a) T 7 o lb. steam at 75 lb. absolute 

= 1175'8X0-70 = 823-06 

(b) A lb. water at 307'43° F. from 39° F. 

=277-44X0-30= 8323 



906-30 

Difference 275*60 

or 76*7 per cent of the heat contained in the dry steam. 

Then the pound of wet steam would only possess about 
three fourths the heating effect of dry steam, only 69 per cent 
of the effect in producing power, for the water then does not 



STEAM. ITS GENERATION AND PHYSICAL PROPERTIES 33 

count at all, and in a dye-vat it would go to swell the volume 
of water only. 

The Properties of Saturated Steam 

All our knowledge of the properties of saturated steam is 
practically derived from the studies of Rcgnault, who first 
thoroughly investigated the subject experimentally. 

From these experiments various formulae have been devised 
to fit the results. Rankine, for example, found that the tem- 
perature and pressure relations were to be expressed by the 
following equation : 

i a B C 

logp = A---^, 

where the log is to base 10, or common, and p = absolute pres- 
sure in pounds per square inch and T = absolute temperature 
in Fahrenheit scale or £+461" 2. 

A = 6- 1007, 
log £=3-43642, 

log C = 5- 59873. 

Where T is expressed in centigrade degrees, 

A = 6- 1007, 
log 5 = 3'1812, 
log (7 = 5-0871. 

The heat S, in B.T.U., necessary to raise the temperature of 
1 pound of water from 32° to any temperature t is given 
with very fair accuracy by the equation 

S = t + 0- 00002* 2 + 0- 0000003Z 3 , 
where t= temperature centigrade. 



34 SUPERHEAT AND SUPERHEATERS 

For t on the Fahrenheit scale the equation becomes 

S = *-32 + 0- 000,000,103(*-39) 3 . 

The latent heat is found approximately as follows: 

L = 1115— 0*7* on the Fahrenheit scale, or 
L= 607-0*7* " " centigrade scale. 

The total heat of evaporation is H = 1082 + 0' 305* (Fahr.) and 

H = 606- 5 + 0-305* (cent.) 

Temperature is that property of heat by which it affects 
bodies so that they can communicate heat to bodies at less 
temperature. 

A degree of temperature F. = 1/180 of the difference between 
the temperature of melting ice and water boiling at the sea- 
level at 30 inches of mercury pressure or 1 normal atmosphere. 

A centigrade degree is 1/100 of the same range. Hence 

i°c.=rs°F. 

The evaporation of water is used as a measure of heat. 
For purposes of comparison, evaporation of water from one tem- 
perature to steam at another temperature is always reduced 
to equivalent evaporation from and at 212° F. 

The equivalent evaporation from and at 212° = -— — , 

966 

where H= total heat as per steam-table and F = temperature of 
feed. 

Thus, to take an example, a boiler evaporates 7| pounds 
of water from feed-water supplied at 60° F. to steam at an 
absolute pressure of 100 pounds. Then the equivalent evapora- 
tion from and at 212° F. will be as follows: 

--1181-87 -(60 -32) Q . OA , 

7 5 t^ = 8 96 pounds. 

yob 



STEAM. ITS GENERATION AND PHYSICAL PROPERTIES 35 

Had the steam been superheated 200° F., the equivalent evapora- 
tion would then have been 

- c 118r7-(60-32) + (200X0-48) 

r5 — -^ - y =9' 69 pounds. 

966 

When we speak of absolute pressure we mean the total 
pressure above zero. Thus one atmosphere of pressure is 14" 7 
pounds at the sea-level, and steam produced in an open vessel 
has to overcome the atmosphere. It is said to have a pressure 
of 14' 7 pounds (or approximately 15 pounds) absolute or 1 
atmosphere, and its gage-pressure is pound. Gage- or boiler- 
pressure is the pressure of steam above 1 atmosphere. There- 
fore gage-pressure plus 14" 7 = absolute pressure. A perfect 
vacuum has pressure absolute and a. minus pressure by gage 
of 14*7, or it is written -14 - 7 

Fig. 5 is a diagram showing the relation between pressure 
and temperature of saturated steam, and Table III gives the 
relation between the temperature and pressure in figures. 



36 SUPERHEAT AND SUPERHEATERS 

DIAGRAM 

Shewing relationship between Pressure and Temperature 
in Saturated Steam. 






o 

2 



T 


ij :; x —i" t 




" iri^ ■ ■ 7 








i 


± t 






x i 


r 


j 


700' Z 




J 


7 


r 


600 4 


t 


r 


^t J 


t 


500 , 


7 


t 


7 


c 


400 .i 


f 


1 


f 


7 


300 -j 


1 


1 




. - / 




/_ 




/ 


^ 


ioo y 


.-^ 


^ 









Temperature in degrees Fahrenheit. 

Fig. 5 






STEAM. ITS GENERATION AND PHYSICAL PROPERTIES 37 



Table III 
SATURATED STEAM * 

TEMPERATURE-PRESSURE TABLE 





Absolute 




Absolute 




Absolute 




Absolute 




Absolute 


Temp . 


Pressure 


Temp. 


Pressure 


Temp. 


Pressure 


Temp. 


Pressure 


Temp. 


Pressure 


F. 


in lb. 


F. 


in lb. 


F. 


in lb. 


F. 


in lb. 


F. 


in lb. 




per sq .in . 




per sq.in. 




per sq. in. 




per sq. in. 




per sq. in. 


60 


26 


100 


0-94 


140 


2-88 


180 


75 


220 


17-2 


61 





26 


101 


97 


141 


295 


181 


7-7 


221 


11 5 


62 





27 


102 


100 


142 


303 


182 


7-8 


222 


179 


63 





28 


103 


103 


143 


311 


183 


8-0 


223 


182 


64 





29 


104 


106 


144 


319 


184 


8-2 


224 


186 


65 





30 


105 


109 


145 


327 


185 


8-4 


225 


18 9 


66 





31 


106 


113 


1*6 


3 35 


186 


8-6 


226 


193 


67 





32 


107 


116 


147 


344 


187 


8-8 


227 


19-7 


68 





33 


108 


119 


148 


353 


188 


89 


228 


20 


69 





35 


109 


1-23 


149 


362 


189 


91 


229 


20 4 


70 





36 


110 


1-27 


150 


37 


190 


93 


230 


20-8 


71 





37 


111 


1-30 


151 


3-8 


191 


95 


231 


212 


72 





38 


112 


1-34 


152 


39 


192 


97 


232 


216 


73 





40- 


113 


1-38 


153 


40 


193 


100 


233 


220 


74 





41 


114 


1-42 


154 


41 


194 


10 2 


234 


224 


75 





42 


115 


146 


155 


42 


195 


10 4 


235 


22-8 


76 





44 


116 


1-50 


156 


43 


196 


106 


236 


233 


77 





45 


117 


1 55 


157 


4 4 


197 


108 


237 


237 


78 





47 


118 


1-59 


158 


45 


198 


111 


238 


24- 1 


79 





49 


119 


1-64 


159 


46 


199 


113 


239 


246 


80 





50 


120 


168 


160 


4-7 


200 


115 


240 


250 


81 





52 


121 


1-73 


161 


4-8 


201 


11-8 


241 


255 


82 





53 


122 


1 78 


162 


5 


202 


120 


242 


259 


83 





55 


123 


1-83 


163 


51 


203 


123 


243 


26 4 


84 





57 


124 


1-88 


164 


5 2 


204 


125 


244 


26 9 


85 





59 


125 


1-93 


165 


53 


205 


128 


245 


274 


86 





61 


126 


1-98 


166 


55 


206 


13 


246 


27-8 


87 





63 


127 


204 


167 


56 


207 


13'3 


247 


28-3 


88 





65 


128 


2 10 


168 


57 


208 


136 


248 


28-9 


89 





67 


129 


2 15 


169 


59 


209 


138 


249 


294 


90 





69 


130 


221 


170 


60 


210 


141 


250 


299 


91 





71 


131 


2 27 


171 


61 


211 


144 


251 


30-4 


92 





74 


132 


2 33 


172 


63 


212 


147 


252 


30-9 


93 





76 


133 


2 40 


173 


64 


213 


15 


253 


31*5 


94 





78 


134 


246 


174 


66 


214 


15 3 


254 


32-0 


95 





81 


135 


2 52 


175 


67 


215 


156 


255 


32-6 


96 





83 


136 


2 59 


176 


69 


216 


159 


256 


332 


97 





86 


137 


2 66 


177 


70 


217 


162 


257 


337 


98 





89 


138 


2 73 


178 


7-2 


218 


16 5 


258 


34-3 


99 


0-91 


139 


2-80 


179 


73 


219 


16 9 


259 


34-9 



* Professor Pullen, "Tables and Data," Scientific Publishing Co., Man- 
chester. 



38 



SUPERHEAT AND SUPERHEATERS 

Table III — (Continued) 
SATURATED STEAM 

TEMPERATURE-PRESSURE TABLE 





Absolute 




Absolute 




Absolute 




Absolute 




Absolute 


Temp. 


Pressure 


Temp. 


Pressure 


remp . 


Pressure 


Temp. 


Pressure 


Temp. 


Pressure 


F. 


in lb. 


F. 


in lb. 


F. 


in lb. 


F. 


in lb. 


F. 


in lb. 




per sq.in. 




per sq.in. 




per sq. in. 




per sq. in. 




per sq 


in. 


260 


355 


300 


67 2 


340 


1184 


380 


196 3 


420 


308 


9 


261 


361 


301 


68-2 


341 


120 


381 


198-7 


421 


312 


3 


262 


36 7 


302 


693 


342 


121-6 


382 


201-1 


422 


315 


6 


263 


374 


303 


70-3 


343 


123 3 


383 


203-5 


423 


319 





264 


38 


304 


714 


344 


124-9 


384 


205 9 


424 


322 


4 


265 


38-6 


305 


724 


345 


126 6 


385 


208-3 


425 


325 


9 


266 


393 


306 


73 5 


346 


128 2 


386 


210 8 


426 


329 


3 


267 


399 


307 


746 


347 


129 3 


387 


2133 


427 


332 


8 


268 


40 6 


308 


75-7 


348 


1316 


388 


2158 


428 


336 


4 


269 


413 


309 


76-8 


349 


133 4 


389 


218 3 


429 


339 


9 


270 


420 


310 


77-9 


350 


135- 1 


390 


220 9 


430 


343 


5 


271 


42-7 


311 


791 


351 


136-9 


391 


223 5 


431 


347 


1 


272 


434 


312 


80-2 


352 


138 7 


392 


226 1 


432 


350 


7 


273 


44 1 


313 


814 


353 


140 5 


393 


228-7 


433 


354 


4 


274 


44-8 


314 


82-6 


354 


142 3 


394 


2314 


434 


358 





275 


455 


315 


83;8 


355 


144-1 


395 


234-0 


435 


361 


7 


276 


463 


316 


850 


356 


146-0 


396 


236 7 


436 


365 


5 


277 


47 


317 


86-2 


357 


147-8 


397 


239 4 


437 


369 


2 


278 


47-8 


318 


87-5 


358 


149 7 


398 


242-2 


438 


373 





279 


48-6 


319 


88-7 


359 


1516 


399 


245-0 


439 


376 


8 


280 


493 


320 


90 


360 


153 6 


400 


247-8 


440 


380 


7 


281 


50 1 


321 


913 


361 


155 5 


401 


2506 


441 


384 


6 


282 


509 


322 


92 5 


362 


157 5 


402 


253 4 


442 


388 


5 


283 


517 


323 


93 9 


363 


159 5 


403 


256 3 


443 


392 


4 


284 


526 


324 


952 


364 


1615 


404 


259 2 


444 


396 


4 


285 


534 


325 


96 5 


365 


163 5 


405 


262-1 


445 


400 


4 


286 


542 


326 


979 


366 


165 5 


406 


265-0 


446 


404 


4 


287 


551 


327 


99 2 


367 


167 6 


407 


268 


447 


408 


4 


288 


56 


328 


100 6 


368 


169 7 


408 


271-0 


448 


412 


5 


289 


56-8 


329 


1020 


369 


1718 


409 


274-0 


449 


4166 


290 


57-7 


330 


103 4 


370 


173-9 


410 


277-1 


450 


420 7 


291 


58-6 


331 


104 9 


371 


176-1 


411 


280-1 






292 


595 


332 


106 3 


372 


178-2 


412 


283-2 






293 


605 


333 


107-8 


373 


180-4 


413 


286 3 






294 


614 


334 


109 3 


374 


182-6 


414 


289-5 






295 


623 


335 


1107 


375 


184 9 


415 


292-7 






296 


633 


336 


112-2 


376 


187-1 


416 


295 9 






297 


643 


337 


113-8 


377 


189 4 


417 


299-1 






298 


652 


338 


1153 


378 


191-7 


418 


302 • 3 






299 


662 


339 


1169 


379 


194 


419 


305 6 







CHAPTER V 



SUPERHEATED STEAM: ITS PROPERTIES. GENERAL RESUME 
OF THE SUBJECT OF SUPERHEATING 



It has already been stated that superheated steam cannot 
coexist with water. One or other must disappear, and this 
will take place the more quickly as the two fluids are agitated 
together. It is said that water may be drained from out of 
pipes in which superheated steam is present. If so, it is because 
there is probably a dividing stratum of saturated steam and 
little violent agitation to produce mixture. With agitation 
and mixture it is not probable that water and superheated 
steam will coexist. 

Let us state briefly what superheated steam is. 

Superheated Steam 

When that critically delicate fluid saturated steam is 

removed from the presence of water and heat is applied to it, 

it loses its characteristics. It will part with some of that heat 

and yet will remain a perfect gas. Superheated steam is thus 

a perfect gas so long as its temperature is higher than that 

of saturated steam of the same pressure. The addition of 

heat in the absence of water causes an increase of temperature 

and of pressure. 

39 



40 SUPERHEAT AND SUPERHEATERS 

The relation between the pressure and volume of super- 
heated steam or steam-gas is given by the equation 

pv =0*64967 7 -22-58 Vp t (1) 

where T = absolute temperature. 

The adiabatic curve of expansion for saturated steam as given 
by Zeuner is 

pv n ■■= constant, 

where the value of the exponent n = 1/035 + 0' la;, where x = 
fraction of a pound turned to steam. 

For superheated steam the curve of adiabatic expansion is 
p^i-333 = constant, showing that the curve drops quickly. 

Since superheated steam is a gas, its volume will expand as 
heat is added to it and shrink as heat is abstracted; but so 
long as the temperature is greater than that of saturated steam 
at equal pressure there will be no condensation but the steam 
will retain its gaseous state. 

The volume of steam is increased by superheating at a 
steady pressure, and the weight per cubic foot becomes less 
per pound. Therefore superheated steam contains more 
heat than saturated steam, but it contains less heat per cubic 
foot because the coefficient of expansion is greater than the 
heat-capacity coefficient. It is argued by some that super- 
heated steam gains some advantage from the increase in volume 
and the reduced heat per cubic foot, but there appears no 
sound reason to think this is the case, since steam is only the 
vehicle of heat and is not itself the working agent. Heat 
endows the molecules with greater activity and they move more 
freely, but they exist in fewer numbers and cany less heat in 
a given volume. 

Knowing that superheated steam contains more heat than 
is necessary to maintain it in the state of steam, it is easy to 






SUPERHEATED STEAM 41 

see that it may be employed to carry heat to certain places, there 
to be employed for some especial purpose, and that after the 
steam has parted with its superheat it still remains as dry 
saturated steam containing the maximum possible heat per 
cubic foot and therefore capable of a maximum amount of 
duty. 

According to the theory of the steam- or other heat-engine 
the efficiency of the working fluid depends upon the range of 
temperature through which the working fluid passes in its 
cycle of changes. The thermodynamic equation usually 



T x -T, 



cited as the theoretical measure of efficiency is E=-- 

T\ 

where T\ and T 2 are the initial and final temperatures absolute 
of the steam. Let these be 820° and 560° respectively for satu- 
rated steam in a chosen example. Then we have 

820-560 260 
*— So"— ST 0317 ' 

Now let us add 140° of superheat and we get T = 960°, whereas 
T 2 remains as before at 560°. Then 

960-560 _400 
E 960 9«T° 416 ' 

the economy of the engine being increased 28 per cent. Now 
with a superheat of, say, 100° and an initial pressure of 50 pounds 
gage-pressure, or, say, 300° F. =760° abs. F., the two values 
for E will be 

760-560 =0 . 263 and 860-560. 

/ 60 860 

the economy being 32 per cent. With half the superheat the 
economy appears to be only 17 or 18 per cent, and when Rankine 
wrote his book on the steam-engine he treated the subject 



42 



SUPERHEAT AND SUPERHEATERS 



entirely thermodynamically, and he found that the economy 
due to superheat was theoretically just about what was obtained 
in practice. Rankine paid little or no attention to the question 
of cylinder condensation. He treated the steam-engine just 
as though the cylinder were inert and did not abstract heat from 
or return it to the working fluid. This regrettable oversight 
destroys much of the value of his investigation of the steam- 
engine, for the economy he found as the theoretical result of 
superheating just happened to coincide closely with the actual 
economy found in practice, which was the result of a quite 
fortuitous combination or compromise of pressure, cylinder 
loss, etc. The coincidence was purely fortuitous and may have 
deceived the discrimination of even so great a man and engineer 
as Rankine. Let it be here directly stated that the thermo- 
dynamic equation in no wa}^ is concerned with the practical 
use of superheat. The results obtained are purely practical and 
to be explained by practical reasoning alone, as will shortly 
be seen. 

The remainder of this chapter is largely a resume of the 
subject of superheating which, at the risk of repetition of what 
has gone before, is offered as a brief expose of the subject. 



Steam produced in the presence of water is said to be 
saturated. It contains the maximum possible weight of 
evaporated water per cubic foot and the maximum amount 
of heat at the pressure of the temperature of the water from 
which it is produced. Saturated steam is dry, it contains 
no un evaporated water, and is invisible. If steam is visible 
or contains watery particles, it is "wet." 

The pressure and density of saturated steam in the boiler 
have a fixed standard of relation to the temperature, only one 
pressure being possible for any given temperature. One 
cannot change without the corresponding change in the other. 



SUPERHEATED^ STEAM 43 

If the temperature falls, there will be condensation of part of 
the steam, the remainder expanding to a less density and 
pressure. Increase of temperature, on the contrary, means 
that more water is evaporated, which leaving the water for 
the steam-space already full must find room, with the conse- 
quence that compression of the mass takes place, and this is 
called rise of pressure. The density of the steam is now 
greater, thus: 

1 lb. of water at 327*7° F.= 0*0180 cubic feet. 
1 " " steam " 100 abs. = 4*34 
1 " " " " 14-7 abs. =26-37 

Steam-pipe Action. — When passed .along pipes, heat is 
lost through the pipe body, and thence by radiation to the 
atmosphere; condensation ensues, the first heat to be given 
up by the steam being the " latent" heat. The mass of 
steam and water shows no marked loss of temperature, until 
in process of 'time the friction of the wet steam, further con- 
densed, produces loss of pressure, density, and temperature. 
The relations of pressure, density, and temperature were 
experimentally found by Regnault, whose figures are still 
accepted as substantially accurate. 

Tables of the properties of steam show that the higher the 
temperature of saturated steam rises, the more rapid is the 
rate of increase of pressure per degree Fahrenheit. 

Thus steam at 20 pounds absolute =227*96° F. 

At 30 pounds the temperature is 250 "29° F., a difference 
of 22' 33° F., or an average of 2* 23° F. per pound increase of 
pressure. At 200 lb. abs. the temperature is 381*64° F., and a 
rise of temperature of 22*33° F. shows a pressure increase of 
59, or 0*38° F. average rise per pound of pressure increase. 

Specific heat, i.e., the coefficient of thermal capacity of 
saturated steam, is 0*305 British thermal unit per pound, i.e., 



44 SUPERHEAT AND SUPERHEATERS 

to raise the temperature 1° F. the thermal capacity of each 
pound of steam is increased by 0*305 British thermal unit. 
Steam-tables show that as the sensible heat of the water 
increases, the latent heat of the steam diminishes. 

Thus a reference to the steam-tables shows the following 
figures : 

Abs. Pressure. Temp. Sensible Heat. Latent Heat. Total Heat. 

301b.... 250-29° F. 219 23 939 02 1158 28 B.T.U. 

2001b.... 38164° F. 353 77 844 57 1198 34 B.T.U. 

+ 131-35 D F. +134-51 -9445 +40 06 B.T.U. 

Then 40*06- 131-35 =0' 305 British thermal unit per degree. 

Superheated Steam cannot live long in presence of water, nor 
can water exist where superheated steam is : they are so intensely 
sympathetic that in contact the}^ unite until one or the other 
is absolutely killed through absorption by its more powerful 
affinity. Superheated steam is, therefore, produced by heating 
saturated steam away from water in separate vessels. It is 
raised to a higher temperature and to a larger volume than 
saturated steam, and contains more heat per pound but less 
per cubic foot. The superheating vessels being in communica- 
tion with the boiler by a sufficient passage, there is no increase 
of pressure by superheating, even when the flow of steam is 
arrested; that portion of the steam which may expand back- 
wards into the boiler will promptly return to saturation con- 
ditions. The added temperature above that of saturation 
is expressed in degrees of superheat, and these degrees multi- 
plied by the specific heat of superheated steam equal the measure 
of the superheat in British thermal units necessary to effect 
the rise in temperature. 

Steam behaves as a " perfect" gas when superheated. It 
must increase either in volume or in pressure, and if super- 
heated in a closed vessel the rise of pressure would be very 
rapid. This, of course, does not occur under the conditions 



SUPERHEATED STEAM 45 

of practice, for in all cases the superheater is simply an exten- 
sion of the boiler steam-space formed by loops, or a series 
of loops, of pipes placed in a combustion-chamber or in the 
flues of a boiler so as to absorb heat from the furnace-gases. 

That superheated steam cannot ordinarily exist in presence 
of water at once incidentally points the impossibility of attempt- 
ing to superheat steam by passing hot gases through pipes ex- 
tended in the boiler steam-space. The density of super- 
heated steam or weight per foot is less than saturated steam, 
or the volume in cubic feet per pound is greater. The density 
diminishes as the temperature rises, but at a diminishing 
rate of growth, the exact ratio of volume increase to temperature 
not having yet l^een accurately determined. The specific heat 
of superheated steam is usually given as 0*4805, or 0*4805 
British thermal unit added to one pound of superheated steam 
will raise its temperature by 1° F. This is now generally 
recognized to be correct for one pressure only, and much below 
the true value, which varies with the pressure and temperature 
of the steam. 

Referring again to the tables of saturated steam. At 
100 pounds abs. the temperature is 327*7° F., and the total 
heat is 1181*87 units made up of sensible heat, 298*09 British 
thermal units occupied in raising the water from 32° to 327*79°: 
and of latent heat, 883*77 units required to evaporate water 
from and at 327*7°. To raise this pound of saturated steam 
from 327*7° to 528° at the same pressure requires the addition 
of 200X0*4805=96*1 British thermal units, calculating on usual 
values. The volume of saturated steam at 100 pounds is 4*34 
cubic feet. 

Raised to 528° F., the volume will be 5*50 cubic feet, or, 
say. 25 per cent above saturation volume. 

Boilers with ample steam-space, at rest and under pressure, 
contain practically dry steam in the steam-space. When 



46 SUPERHEAT AND SUPERHEATERS 

delivering steam and receiving furnace-heat, the water sur- 
face is agitated and throws off much water into the steam- 
space, and some of this gets away with the steam. No boiler 
delivers dry steam, and all claims to this effect are based on 
fallacy of some sort. When much water is carried over, the 
boiler is said to prime. 

Priming is caused by (1) excessive evaporation or forcing, 
the boiler being too small; (2) insufficient water area, so that 
too much steam rises per unit of area; (3) restricted steam area, 
the steam having too sweeping an effect; (4) defective circula- 
tion; (5) rapid changes in the draft of steam or sudden demands 
for large volumes of steam; (6) dirty water, which causes 
foaming. It is not possible so to collect samples of steam as 
to show accurate calorimetric results of a boiler performance. 
The abnormally high evaporation recorded by some tests may 
be set down to priming. 

Wet steam is most wasteful. Lancashire boilers, which 
usually give the driest steam, down, probably, to 3 per cent 
of moisture, may occasionally give steam as much as 20 per 
cent wet. According to Tyndall, a gas containing moisture 
will absorb and reject heat more readily than if dry. This 
fact partially accounts for the economy of superheat. 

With all steam, heat begins to radiate at once on leaving 
the boiler. Saturated steam in bare iron pipes has been 
calculated to lose 2\ British thermal units per hour per square 
foot per degree of temperature head. This rate will increase 
by wetness. Even with covered pipes the heat loss is serious, 
and in practice saturated steam arrives very wet at its working 
point, with a fall also of pressure and of temperature. This 
is particularly noticed in the bleaching, drying, and printing 
trades, in soap and sugar factories, and similar industries. 
Irregular and sudden demands call up much priming water, 
and the pipes are of great length with many branches. In 



SUPERHEATED STEAM 47 

this way steam dilutes the liquors into which it enters out of 
all ratio to the heat it puts into them. The water adds very 
little heat, and yet it represents so much coal, and is, therefore, 
a dead loss. Such wetness may average 30 per cent. 

But the evil does not stop at this primary useless con- 
sumption of excess fuel, as will be seen later. Let an example 
be taken of steam at 100 pounds abs., 30 per cent wet and 
reduced 20 per cent. 

B.T.U. 

1 lb. of dry steam should contain 1181'90 

Then T V lb. steam at 75 lb. abs. pressure contains 823'06 

And T 3 o lb. water (at 307'43° F.) contains 83*23 

Total i 906-3 

or a loss of 275' 6 British thermal units, a total heat only 76*7 
per cent of that of the original steam for purposes of boiling, 
and only 69 per cent for engine purposes. The evil is the cause 
of all manner of devices, such as steam-traps and water-separa- 
tors, but they can never give us steam wholly purged of wetness. 

The specific heat of superheated steam is usually accepted 
as 0*4805 at constant pressure and 0*346 at constant volume, 
and the calculations are all based upon these values. For 
temperatures of superheat between 150° and 300° and at about 
185 pounds abs. pressure, Mr. Cruse considers that superheated 
steam has probably a specific heat of 0*650 at constant pressure 
and 0'480 at constant volume, and that the specific heat rises 
with the pressure and temperature in conformity with other 
gases whose specific heat at high temperature is several times 
the normal. 

Returning to our pound of steam at 100 pounds abs. and 
327' 70° F. with a volume of 4*34 cubic feet: to superheat this 
weight through 200° F. to 528° and to a volume of 5*5 requires 
96' 1 British thermal units. The total heat is now 1181*9 + 96*1 



48 SUPERHEAT AND SUPERHEATERS 

= 1278 British thermal units, or an addition of 8*13 per cent. 
Now this is not an addition to the work energy per unit volume. 
If, instead of adding this heat as superheat, we had added it 
to the boiler, we should have found that steam with 1278 
British thermal units per pound would have exerted a pressure 
of about 2000 pounds at a temperature of 642*7° F., and at 
a volume per pound of only 0'26 cubic feet. Superheated 
steam has, therefore, less heat per cubic foot than saturated 
steam, and its economy is purely connected with the question 
of dryness. 

The work done by the superheater consists of two portions: 

(1) the evaporation of the wetness of the steam supplied to it; 

(2) the superheating of the dried steam. Thus it requires 
53' 03 British thermal units to dry 1 pound of steam 6 per cent 
wet. 

Superheated steam will carry through pipes with less loss 
than saturated steam. In carrying it through pipes it does 
lose heat, but it does not and cannot condense until it has lost 
so much heat as to convert it to the normal saturated condition. 
Saturated steam cannot lose any heat without the formation 
of some wetness. Superheated steam avoids the evils of water- 
hammer and leaking joints in steam-pipes, but it renders 
useless such joint-rings as I.R. or other vegetable or animal 
substances, and demands copper or asbestos alone or combined. 

The main object of superheat, to those who understand it, 
is to avoid the loss of latent heat energy and expansive power 
suffered by saturated steam by condensation in the pipes and 
in the engine cylinder. The degree of superheat is deter- 
mined by the class of engine and by the conditions of work; 
and no absolute rule is possible, nor can rule-of -thumb tables 
be relied on which give so much economy for so many degrees 
of superheat. Suffice to say that absolute dryness is often 
the biggest gain, and the first 100° of superheat is much more 



SUPERHEATED STEAM 49 

valuable than any other 100°, and in all but extreme cases 
a superheat of 200° will carry steam diy to its work and super- 
heated, thus insuring to it the full value of its thermal capacity. 
When boilers are near the engines, the steam may continue 
superheated to cut off at | with no initial cylinder condensation. 
The usual composite steam which enters the cylinder meets 
surfaces of metal that have been cooled by contact with wet 
exhaust-steam at condenser temperature, and by loss of heat 
in evaporating water off the cylinder surfaces. This process 
is cumulative up to such a point that no more water can lodge 
in the cylinder without being too promptly exhausted to permit 
it to evaporate at the expense of cylinder heat. A vicious 
cycle of events is thus set up. Add superheat to the initial 
steam and the warming up of the cold cylinder is effected by 
the superheat. Condensation is reduced, less water is re- 
evaporated on the exhaust, and the cylinder is hotter to begin 
the next stroke. Here again superheat has a cumulative effect, 
and very much less heat than at first appears necessary will 
effect a big result, for with dry 7 steam the cylinder is rendered 
more nearly adiabatic, and the reduction of water progresses 
with each stroke until a balance is obtained. Water multiplies 
the rate of heat exchange between the metal and the working 
fluid; superheat diminishes the rate of exchange. If there 
is sufficient superheat to carry the steam dry to cut-off, the 
only liquefaction in the cylinder will be due to the conversion 
of latent heat energy into work during expansion, and only 
this wetness remains to be re-evaporated. The cylinder is 
maintained hotter, and every accession of heat reduces the 
condensing effect. Superheat supplies the entering steam 
with the heat to re-heat the cylinder metal without condensa- 
tion during admission, or material drop of pressure. The 
added heat minimizes liquefaction during expansion and 
enables the original latent heat of evaporation to perform 



50 SUPERHEAT AND SUPERHEATERS 

more work. It therefore adds to the work efficiency of the 
steam per pound supplied to the cylinder. 

Some engineers have claimed that the large increase of 
volume due to superheat is a factor which makes for higher 
efficiency and economy, but this is a claim that cannot be 
supported. Thus, to revert to our standard example of 1 
pound steam at 100 pounds abs. and 327' 7°, with a total heat 
of 1181"3 units and a volume of 4*34 feet, an addition of 96' 1 
heat-units will cause a superheat of 200°. An addition of 
8' 13 per cent of heat has added 25 per cent to the volume. 
A cubic foot of saturated steam represents 272*35 British 
thermal units; a cubic foot of superheated steam contains only 
235*55 British thermal units, or 13^ per cent less heat per 
cubic foot. But to develop power from heat it is desirable 
to use a fluid of maximum heat capacity per cubic foot. Hence 
the economy of high pressure and the multiple expansion 
it renders possible. 

With saturated steam the pressure and density increase 
with the temperature. This is reversed by superheat, for 
the higher the superheat the greater the volume and the less 
will be the density. This shows less heat per unit volume 
and less expansive power. As already shown, the rate of 
volume increase decreases as pressure rises, and the rate of 
increase falls with rise of temperature. 

Therefore, at present-day practical temperatures, less 
energy is carri^xl into the cylinder by a cubic foot of super- 
heated steam than by a cubic foot of dry saturated steam. 
If, therefore, steam did remain superheated up to cut-off, it 
would l!e necessary, for a given power, to cut off steam admis- 
sion at a later point or to use larger cylinders. But the inter- 
action of the working fluid and the cylinder metal already 
referred to steps in here. Superheated steam, to begin with, 
enters the cylinder more easily than saturated steam, and, 



SUPERHEATED STEAM 51 

where the superheater is properly constructed, nearer to the 
boiler-pressure. As the piston moves forward the metal of the 
cylinder takes up the surplus heat, and when the cut-off point 
is reached the temperature is usually at about saturation-point 
and the cylinder is diy. There is, therefore, the fullest possible 
weight of steam present in the cylinder at cut-off, and con- 
taining the maximum heat-energy of saturated steam. Up 
to this point the area of the indicator-diagram is a little larger 
than if no superheat had been used, because the steam is more 
lively. As the superheat is lost in the cylinder metal, its 
place has been filled by further steam from the boiler, and the 
expansion-curve which follows should not vary far from the 
hyperbola. With saturated steam and considerable initial 
water and condensation in the cylinder, it is true that the curve 
does rise above the hyperbola, but this occurs chiefly at the 
toe of the diagram, and represents re-evaporation of the water 
by the cylinder-walls — a most wasteful cycle of operations carried 
on also during the exhaust-stroke at the expense of the initial 
steam. All these losses are minimized by means of superheat. 
The increased area of the toe of the diagram is more than 
balanced by the reduction of initial area and by exhaust back- 
pressure. 

Important, therefore, as is the role played by superheat, 
let us not misinterpret its action. Thermodynamical ques- 
tions do not enter into the argument. Stated briefly, we 
superheat steam as a part of the process of supplying to an 
engine steam that is just above saturation temperature at 
cut-off. Automatic engines will govern the admission to fit 
the power developed. The cut-off will vary but little either 
way, but the weight of steam and water present at cut-off 
will always be less. No advantage accrues from increased 
volume, and such increase is removed during admission by the 
absorptive power of the cylinder metal and of the moisture 



52 SUPERHEAT AND SUPERHEATERS 

which all our care cannot wholly remove from the exhaust 
end of the cylinder. Dependent, of course, on the allowable 
maximum temperature, it might possibly be an advantage to 
carry so much superheat as to carry steam superheated along 
the expansion-curve: although Mr. Cruse says this is not 
possible in actual work. A temperature of 550° is about 
the maximum permissible in a working cylinder, or, perhaps, 
only 500°. If steam at a higher temperature were procurable, 
it might be admitted byway of the encircling jacket with advan- 
tage, and possibly in this way dry steam might even reach the 
low-pressure cylinder of a compound engine. Watt always 
said that a cylinder should be as hot as the steam which enters 
it, and he was not far wrong. Highly superheated steam in 
the jacket is not like the same steam in the cylinder. The 
obvious limitations of superheat are, first, the superheater 
itself, and next, the cylinder, piston, rods, packings, and 
valves and lubrication, all of which set an earlier limit on 
superheat than should be set by the turbine engine with its 
freedom from rubbing parts. 

From what has been said above it is clear that superheat 
cannot be substituted for high pressure. Pressure is the factor 
in the cylinder, and superheat, great as are its benefits, is the 
preserver of pressure and latent heat. If we could prevent 
loss of heat in steam-pipes and find the ideal "adiabatic" 
cylinder, we should more usefully employ the heat of super- 
heat in producing saturated steam at higher pressure. With 
a full appreciation of the advantage of high superheat we 
must not run outside the border of common sense. Cast-iron 
cylinders will not work red-hot, nor will any advantage accrue 
from degrees of superheat even far below a red heat. Any 
advantage from expansion of steam is increased by superheat. 
High-pressure steam demands superheat as urgently as does 
low-pressure steam. It is much denser and carries wetness 



SUPERHEATED STEAM 53 

probably more easily when at 300 pounds than when at 100 
pounds pressure. While high steam-pressures spell economy, let 
us not forget that they also spell wear, tear, maintenance cost, 
and other discounting circumstances, combined with heavy 
boilers and generally heavy first cost. It therefore behooves 
engineers to avoid extremes, and to try whether moderately 
high pressures and moderately high superheating cannot be 
combined to give the best commercial economies. 

There are two main types of superheater : flue and separately 
fired. The former are placed in the flues of the boiler and the 
hot gases pass through them. The}^ are most readily placed 
in the downtakes of Lancashire and similar boilers, which have 
large water and steam capacity, convenient combustion- 
chambers, and external flues. The temperature of the gases 
at the back of such boilers is about right for the superheater. 
The average water-tube boiler is less easily fitted with a 
satisfactory superheater. If the boiler is worked at an eco- 
nomical rate, the superheat will be insignificant. If the boiler 
is fired sufficiently to get superheat, the boiler efficiency will 
be wastefully low. Marine-type boilers are also unsuitable for 
superheaters. At the back of these short boilers the gases are 
too hot. After the gases have passed the smoke-tubes they 
are too cold. For all such boilers the superheaters must be 
separately fired and, where properly constructed, they will add 
to the steam and water capacity of the boiler plant, which is 
usually far too restricted in case of water-tube boilers. The 
locomotive is also a problem by itself, as shown in the chapter 
on the locomotive. 

In respect of the specific heat of superheated steam there 
is as yet nothing definite known. Many are experimenting 
on the subject, and Professor Carpenter among others has 
done much work. Experiments are also being made at the 
Physical Laboratory at Bushey Park, London. 



54 SUPERHEAT AND SUPERHEATERS 

Professor Carpenter found at atmospheric pressure that the 
specific heat was C P =0 - 46305 + ap, where a is a constant and 
p is the absolute pressure. The results indicated an increase 
of specific heat with increase of pressure, but did not for small 
ranges of superheat appear to show much increase for increased 
temperatures at constant pressure. Berthelot's experiments 
on gases pointed to considerably increased superheat at high 
temperatures, but he did not deal with any temperatures so 
low as practically workable steam. Professor Jones in 1900 
found C P =0-462 + 0-00065p. Mr. Berry found C p = 0-48 + 
0'0056p and indicated an increase with pressure but a decrease 
with increase of superheat temperature. Professor Thomas, 
following the same methods, found 

for 5° of superheat C P =0'48 + 0-0007p 
a 100 o a tt C p =0-48 + 0'0006p 
" 180° " " C P =0-48 + 0'0005p 

again showing increase with pressure but a dminished rate of 
increase with rise of temperature. Here we may leave the ques- 
tion, for it were useless to discuss it further, since so little 
that is definite is yet known. For ordinary practice the author 
has thought well to calculate on a basis of a specific heat of 
0'55 to 0-60. 

Table IV, Specific Volume of Superheated Steam, is com- 
puted from Schmidt's formula based on Hirn's experiments, as 
follows : 

441 -44- T 
&=0-59276X p , 

where S v = specific volume in cubic feet per pound; 

T — temperature of saturated steam + superheat; 
P = absolute pressure in pounds per square inch. 



SUPERHEATED STEAM 



55 



Table IV 
SPECIFIC VOLUME FOR DEGREES OF SUPERHEAT 





£ 6 

t- 3 


Temperature Fahrenheit. 




53 


20° 


40° 


60° 


80° 


100° 


120° 


140° 


160° 


180° 


200° 


70 


614 


6 47 


664 


6-81 


6 98 


7-15 


7 32 


749 


7 66 


7-83 


8 00 


80 


5 


42 


5 


72 


5 


8S 


6 


03 


6 


17 


6 


32 


6 


47 


6 


62 


6 


77 


6 


92 


7 


07 


90 


4 


86 


5 


IS 


5 


28 


5 


41 


5 


54 


5 


67 


5 


81 


5 


94 


6 


07 


6 


20 


6 


33 


100 


4 


34 


4 


67 


4 


79 


4 


91 


5 


03 


5 


15 


5 


27 


5 


39 


5 


51 


5 


63 


5 


75 


110 


4 


03 


4 


29 


4 


42 


4 


51 


4 


61 


4 


72 


4 


83 


4 


94 


5 


05 


5 


15 


5 


26 


120 


3 


71 


3 


96 


4 


06 


4 


16 


4 


26 


4 


36 


4 


46 


4 


56 


4 


66 


4 


75 


4 


85 


130 


3 


44 


3 


69 


3 


78 


3 


87 


3 


96 


4 


05 


4 


14 


4 


23 


4 


32 


4 


41 


4 


51 


140 


3 


21 


3 


45 


3 


53 


3 


62 


3 


69 


3 


79 


3 


87 


3 


96 


4 


05 


4 


13 


4 


20 


150 


3 


01 


3 


24 


3 


32 


3 


40 


3 


48 


3 


55 


3 


63 


3 


71 


3 


79 


3 


87 


3 


95 


160 


2 


83 


3 


05 


3 


13 


3 


20 


3 


28 


3 


36 


3 


42 


3 


.50 


3 


57 


3 


64 


3 


72 


170 


2 


67 


2 


89 


2 


96 


3 


03 


3 


10 


3 


17 


'3 


24 


3 


31 


3 


38 


3 


45 


3 


52 


180 


2 


53 


2 


75 


2 


81 


2 


S8 


2 


94 


3 


01 


3 


07 


3 


14 


3 


21 


3 


28 


3 


34 


190 


2 


41 


2 


62 


2 


68 


2 


74 


2 


80 


2 


87 


2 


93 


2 


99 


3 


05 


3 


12 


3 


18 


200 


2 


29 


2 


50 


2 


56 


2 


62 


2 


68 


2 


74 


■'2 


80 


2 


86 


2 


91 


2 


97 


3 


03 



According to Griesman the specific heat of superheated 
steam is 

C P =0-00222*. -0-116, 



where t s is the sum of the saturated and superheat temperature. 



CHAPTER VI 

STEAM-PIPES AND -VALVES 

Superheated steam being hotter than saturated steam 
causes very much greater expansions, and in making out steam- 
pipe designs this increased expansion must be provided for. 
Pipes should not be too long between expansion-bends, branches 
from boilers to mains should be sufficiently long to provide 
for lateral movements of the connection to the main, and 
anchorages should be very carefully selected. 

It would often be correct practice to tie the ends of any 
straight main against a pier or other abutment sufficient to 
carry the stresses. Next it would seem proper to stretch all 
the U bends by means of screws, so as to put them under an 
initial pull when cold equal to that of the expansion. When at 
work the expansion would lengthen the pipe and remove the 
initial stress of the tension-ties, but these would remain as a 
safeguard against possible rupture. The place of the end 
abutments may be taken by longitudinal tension-rods with 
some slight spring effect under their end nuts. Such fixed ends 
to a pipe range compel the expansion-bends to act. Usually 
the expansion-bends are left to take care of themselves. Is it 
advisable to let them do this, especially with the increased 
movements due to superheated steam? With end abutments or 
tie-rods the expansion of a pipe due to pressure may be ini- 

56 



STEAM-PIPES AND -VALVES 57 

tially eliminated and each bend will act for heat expansion in 
a proper manner. 

With superheated steam there must be no attempt to use 
cast-iron valves, though with a really scientific attention to 
steam-pipe stresses it cannot, perhaps, be said that cast-iron 
might not be safe. Indeed the ruptures of cast-iron valves are 
perhaps invariably due to pipe stresses of an indeterminate 
order, and against such stresses only design can provide, and 
no designer has yet gone so far as to treat pipes other than 
to leave them to settle among themselves the points where 
stresses may cause yield. Still, under the best of intentions 
cast iron is doubtful and steel has largely taken its place. Cast 
steel is strong but is apt to be badly cast, to contain blow- 
holes, and to be unreliable. Cruse employs pressed-steel plate, 
making of his valves a thorough work of boiler qualit}'. Neces- 
sarily such valves are costly, but modern conditions with 
superheat do certainly appear to justify them, and if made in 
quantity the element of cost ought not to be serious. The 
body, covers, outside pillars, and cross-bar, indeed the whole 
valve, are of steel, the actual valve and seat being a nickel 
alloy of special make. Steam-piping with such all-steel valves 
has then the qualities long asked for it by the author, who 
fails to see the reason for the double pipe and ring main and has 
long argued that a single pipe of the best possible material 
and design is far less liable to fail than the two cheaply put up 
pipes of the ordinary double main. The all-steel valve may 
be made either as a riveted branch body or as a welded body 
when the branches cannot be pressed. The acetylene blow- 
pipe offers facilities for such welding that render a welded 
branch possible with certainty of sound work. 

Copper must not be employed for superheated steam: 
it becomes unsafe at the temperature. 

As regards the effect upon steam-pipe diameters it is generally 



58 SUPERHEAT AND SUPERHEATERS 

admitted that since superheated steam effects an economy of from 
15 to 25 per cent, the steam-pipe need not be made any larger on 
account of the increased volume of the superheated steam. An 
ordinary modern temperature for saturated steam will be 820° abs. 
At 100°, 150°, 200°, 250°, and 300° of superheat, or 920°, 970°, 
1020°, 1070°, and 1120° abs., the relative volume of a given 
weight of steam = 1 vol. at 820° abs. will become 1*12, 1*18, 
1*24, 1*30, 1'36, the volume increasing much at the same rate 
as the economy, though this has no direct connection with 
volumetric increase. Then, since the natural molecular velocity 
of hotter steam must be greater than that of colder steam, 
the pipes may be smaller on this account, probably in the 
inverse ratio of the square root of the respective absolute 
temperatures. Further, the velocity of wet steam is slower 
because it has to carry a burden of inert water, and this again 
makes a smaller pipe sufficient for superheated steam for any 
given power. 

The densities of steam-gas at 820° to 1120° abs. are as 
follows: 100, 91, 85, 81, 77, 74, approximately, and probably 
it will not be far wrong to take these figures to represent the 
relative areas of pipe for a given power, taking into account the 
economy, the increase of volume, the increase of molecular 
velocity, and the increased dryness. Whence the rule would 
evolve that the diameters of pipes should vary as the square 
root of the density. Worked out for the foregoing six tempera- 
tures, a 10-inch pipe for saturated steam at 360° F. =820° abs. 
would become 9J", 9i", 9", 8|", 8J", or commercially 9J", 
9", and 8J". 

As regards the area through the superheater-tubes it is quite 
usual to allow less than that through the main steam-pipe, since it 
is found that with small-tube superheaters the steam is required 
to have a frictional brushing effect and high velocity in order 
to take up heat from the pipes, and this results in a serious 



STEAM-PIPES AND -VALVES 



59 



reduction of the pressure, as much as 15 or 20 pounds of pres- 
sure being lost by the throttling effect of some superheaters. 

In the Cruse controllable superheater the net area through 
the pipes is from 25 to 50 per cent in excess of the area of 
the main steam-supply pipe; and this appears good practice, 
for by means of the larger area the friction is so much reduced 
that the apparatus is practically free from throttling effects. 

Mr. Foster considers that the flow of superheated steam is 
under different laws from those which govern saturated steam, 
and concludes generally that the rate of heat transfer per degree 
of temperature difference per unit of area increases with the 
velocity of flow and more rapidly in small than in large pipes, 
but that the percentage of loss decreases with velocity, since, 
though more heat is transferred, there is an even greater propor- 
tion of steam passing. He advises 6000 to 8000 feet per minute 
for straight runs of pipe for superheat of 100° to 200° F. He 
quotes a German figure for the loss in temperature per 100 feet 
of pipe, for Mr. 0. Berner gives the loss at an average of 176' 5 
pounds pressure and 482° F. or 105° of superheat as follows : 

Table V 
LOSS OF TEMPERATURE PER 100 FEET OF PIPE 



Diameter of 
Pipe, Inches. 


Velocity in Feet per Minute. 


1968 


3936 


5904 


3 " 937 

7 874 

11-811 

15748 


50 3 
255 
170 
126 


25 5 
127 

8-23 
607 


16 45 
8 23 
549 
439 



He gives the diagram of Fig. 6 to illustrate the temperature 
drop in superheated-steam lines with velocity, and Fig. 7 to 



60 



SUPERHEAT AND SUPERHEATERS 



show the variation in heat transfer with velocity. Fig. 8 is 
given to show the cross-sectional area required for passing 



Q 

1 

* 30 
^ 20 

1 




\ 














\ 














\ 
















\ 


^ 
















>-^J 



















1,000 2,000 3,000 4,000 SflOO 6,000 
VELOCITY IN FT. PER MIN. 

Fig. 6 



1000 pounds of steam per hour for different pressures and 
temperatures, the curves A, B, C, D, E, F standing for 
saturated steam and for superheats of 100°, 200°, 300°, 400°, 




I.OOO 2,000 3,000 -4.000 S,O00 6,000 

velocity in ft. per min. 
Fig. 7 



and 500° respectively. Fig. 9 is constructed from figures given 
by Berner for temperature loss with varying diameters and 
velocities. 






STEAM-PIPES AND -VALVES 



61 



None of these diagrams is to be taken as more than an 
approximation to facts, for there is as yet little definite known. 



a i ft W\ 1 1 1 1 




280 XlAXZ - 




m AlX-AA 




g So JAW,- - 




* 2 £2a\1 




g 2 ielJJ: . 




« 2 33nK . 




« is C^JJ^ - 




* 55 i\X$SK - 




^210 iTi ^7rr 




s *» " s:s$3 >5s 




g }g V^S- 


\ 


P 170 X^ VV- 


;s: _ 




-\S^ 




s£*% 


S IS X N ^ 


^^2^ 




"^J^x 




-^o^^ 


S 120 N 


* ^ ^^*^^ 


a jnn 




2 an 


^•v,**^^"^^^^"" 5 '"- 


an 


- ^ss^^^s 


* 




m 


~~~-~. 


1MB, pSHla-i .5. X.J .1110 12 14 


16 118 2.0 2 2 2.4 2,6 


I'sooo & £ j 1 .7 .$ .a l.o lji 


1.2 113 lU 115 116 1|1 1{8 1{9 2102.1 


3w«o .3 .4 -6 .6 .7 4 .9 


1.0 1]1 112 1J3 l.<4 i;5 1161 r 



AREA IN SQUARE INCHES- 



Fig. 8 



As regards the expansion to be provided against, it appears 
practically necessary to assume pipe expansions double that 



I 

fc so 



§ 30 





V 










































* 


\ 


x*"'- 

p 








1 


^5 


b^ 


pajr 



























IfiOO 2flOO 3,000 +poo SJOOO 6,O00 

steam velocity in ft. per min. 
Fig. 9 



found with saturated steam. Needless to say, also, with 
superheat there must be no organic jointing material employed. 



CHAPTER VII 

SUPERHEAT AND STEAM-TURBINES 

It has been claimed for the steam-turbine that, owing to 
the continuous flow through it, every part is always at the 
same temperature as the steam flowing by it. This is true 
only in a limited sense. The parts of the turbine are heat- 
conductors and heat is constantly flowing from the small or 
high-pressure or entrance extremity towards the larger, cooler, 
low-pressure extremity. The initial steam is deprived of heat 
which is sent forward to supply the heat lost by succeeding 
portions. Just what the net effect may be cannot be known. 
On the one hand the fall of temperature of the steam becomes 
less steep, and on the other hand the mass of metal becomes 
greater towards the exhaust end. It may safely be said that 
heat is lost and that some of the heat lost has descended the 
heat declivity and has finally been transferred to the exhaust- 
steam and lost in the condenser. So far there is mere loss 
of heat, which ma}' be classed practically as a radiation loss. 
But there is no variation of temperature at any point in the 
turbine. The internal surfaces of the turbine are enormously 
increased beyond those of the reciprocating engine, and the 
usual effects of temperature range in promoting cylinder 
condensation would be very great if the temperatures in a 
turbine did change so widely at each locus. 



62 






SUPERHEAT AND STEAM-TURBINES 63 

But there is a system of regulation of steam-turbines which 
does introduce the very evil of temperature variation from which 
the turbine has often been claimed to be free. This is the 
system of regulation by gusts, the steam being admitted by 
a valve which is continuously opening and closing, the ratio 
of open and close time being varied. The pressure within the 
turbine is always oscillating, and the low pressure of the con- 
denser endeavors constantly to run up towards the high-pres- 
sure end with each swing of the regulation-valve. Thus, 
from end to end, the metal of the turbine casing, rotor, and 
blades is exposed to a rapid series of temperature changes 
which within their severity must exercise a bad effect upon 
the passing steam by way of condensation and re-evaporation 
with the result that more or less water is being swept through 
the blades to act as a brake on the rotation. A certain degree 
of superheat is therefore essential for the best working of all 
turbines and particularly of those which rely on gust regulation 
instead of upon a steady throttling movement which appears 
perhaps more rational. 

Trials made of a Willans engine at Rugby with the same 
load and vacuum of 28" and a superheat of 0°, 100°, 200°, 
and 260° showed a steam consumption of 14*6, 12" 5, 10*9, 
and 10'0 pounds per B.H.P., the steam-pressure at the throttle 
being 185 pounds and the load full. Thus the economy per 
100° of superheat appears to be 14' 4 per cent for the first 100°, 
12' 7 per cent for the second, and at the rate of 12" 1 per cent 
for the third. 

Superheat is thus a necessity for the steam-turbine, and 
particularly for those which combine a large superficial area 
of blades and a variation of temperature introduced as an 
incident of gust regulation. 

There seems to be no little trepidation on the part of some 
steam-turbine builders as to superheat. Certain difficulties, 



64 SUPERHEAT AND SUPERHEATERS 






such as blade stripping, have been attributed to superheat or 
to sudden or excessive variations of temperature due to uncon- 
trolled apparatus. But the turbine requires superheat to 
enable it to reach its best economy. It would appear, however, 
that a superheat of 140° F. is practically sufficient. It must 
be clearly recognized that a turbine without superheat is merely 
a form of Froude's water-brake, for every ounce of water 
passing through, no matter how finely divided, is quite inert 
material and serves merely to check the running of the machine. 
Hence, though the interchange of heat between the steam 
and the cylinder metal may not be so great as in a reciprocating 
engine, — though this is probably untrue with severe gust 
regulation, — there is always to be reckoned the mechanical 
effect of water — the Froude effect, to coin a term from the 
inventor of the water-brake. 

Speaking generally, a steam-turbine will not benefit by 
superheat as a rule more than one half so much as the ordinary 
reciprocating engine, but on light loads with a large range of 
internal pressure and gust regulation a turbine should, it may 
be supposed, derive more benefit than it will when fully loaded 
and steadily hot throughout. 

There are two main sources of trouble with superheat in 
the turbine. These are the destruction of the blades and 
the unequal expansion from sudden change of temperature. 

Blades of unsuitable brass alloy will not stand the high 
temperature of the steam, and only special alloys or steel can 
be employed. Copper alloys, even if they stand the tempera- 
ture, will usually have an expansion greater than steel, and the 
longer blades may very well acquire temperature so quickly 
when the steam flows hotter that they expand more quickly 
than the massive casing of less expansive steel, and there may 
be contact, rubbing, and even wholesale stripping of blades. 
The shrouding of blade ends in a smooth channel is a safeguard 



SUPERHEAT AND STEAM-TURBINES 65 

in such a case, but it is plain that with superheated steam 
the clearances must be greater or the control of temperature 
of the superheated steam must be so good that sudden changes 
of temperature cannot occur. Such control is possible, but 
only at the expense of the best apparatus. Much can be done 
in the design of the turbine itself. The outer casing should be 
so held to the bed that it will not be liable to expand ovalfy as 
it would be if firmly bolted down by a pair of lateral fins. 
Nor must it be induced to curve in a longitudinal direction by 
rigid attachment at both ends to a colder bed-plate. These 
are matters for the designer who has had to face the same 
problem in the steam-engine wherein sudden changes of superheat 
were communicated first to the lighter parts, and such things as 
piston-valves carried by internal wings would expand into 
polygons and bear hard at points opposite the inner wing- 
carrier. Mass effect will inevitably cause trouble in any steam- 
engine or turbine if varying temperatures are not allowed for 
in construction and in fastenings. 

In the turbine an even temperature is more particularly 
desirable from the fact that it is difficult to provide a greater 
blade clearance without producing an undesirable leakage. 
But the water-brake effect of wet steam and the economy pos- 
sible with superheat are too great to permit of neglect, and 
the engineer must strive to secure superheat so regular that 
a variation of 10° F. may be the limit. 



CHAPTER VIII 

THE BEHAVIOR OF ENGINES WITH SUPERHEATED STEAM 

There is a common belief that when an engine is worked 
with superheated steam it must necessarily be much hotter 
than when saturated steam is employed, but this is probably 
quite erroneous. We have seen, when dealing with the general 
interactions of the steam and the cylinder metal, that all the 
superheat has usually gone out of the steam before the point 
of cut-off has been reached by the piston. Let the course of 
the steam be traced through the engine and its effect observed 
for each part with which it comes in contact. First of course 
comes the valve. This is without doubt exposed to the full 
temperature of the superheated steam, and it must become 
considerably hotter than when exposed to saturated steam. 
Its working faces will probably be dry and their lubrication of 
very little account, for no oil can possess very much viscosity 
at even ordinary temperatures of superheat. 

Practice shows that the Corliss valve cannot safely be trusted 
to work with steam superheated to 500° F. and, especially for 
the higher degrees of superneat, some other form of valve is 
requisite, as to which more will be said later. After passing 
the valve the steam enters the cylinder and encounters the 
piston and piston-rod. The rod is then moving out of the 
stuffing-box and into the steam-space of the cylinder. This 
point is important to note. Had the opposite been the case 



BEHAVIOR OF ENGINES WITH SUPERHEATED STEAM 67 

the highly heated surface of the rod would have entered the 
packing from the steam-space, occupied by superheated steam, 
and trouble might have ensued with the dry packing. But 
it is by no means certain even then that the rod would become 
so very hot, for superheat rapidly disappears. However, it 
will be observed that between the time when the rod begins 
to be exposed to the superheated steam and the time when it 
again enters the packing it is exposed to the expanding steam 
and to the exhausting steam, and that part of the rod which 
may be supposed more likely to be the hottest, viz., the part 
near the piston, is exposed for the longest period of time to the 
colder steam. There seems but little chance of the rod being 
heated up much beyond the temperature of the saturated steam 
present at the time the rod is entering the packing. 

Similarly it is to be observed that the piston when exposed 
to superheated steam is always moving away from those parts 
of the cylinder which have been or are exposed to the super- 
heated steam and is moving towards and upon a part of the 
cylinder from which steam and water have just departed on 
their way to the condenser. So far, then, as concerns the two 
parts, the piston and its rod, very little difference would appear 
to be made, as between the conditions of working, whether the 
steam be saturated or superheated. 

As regards the valves, however, there is undoubtedly a 
difference, and the best modern engines for superheat employ 
valves which have no rubbing surfaces but are merely modi- 
fications of the old Cornish or stamper valve, which opens and 
closes on a seat like an ordinary mushroom stop-valve. For 
ease in movement such valves are made double with the two 
areas very little different, so that there is a slight tendency 
to remain closed. In the Van der Kerchove valve we have 
a return to the piston-valve, used a few years ago very much in 
Lancashire. These valves simply worked to and fro in a cylin- 



68 



SUPERHEAT AND SUPERHEATERS 



clrical shell. For superheated steam they work to and fro 
through the breadth of the steam-port plus a sufficient steam- 
cover, and, like the stamper valve, they drop quickly to cut 
off steam, but they do not drop upon a seat : they merely drop 
past the port-opening in their shell. Here it might be thought 
that there was a rubbing contact with superheated steam, but 
if the circumstances be investigated it will be found that such 
is not the case. 

Steam is surrounding the valve-shell and pressing through 
the ports upon the valve-body. The valve suddenly lifts and 
opens the ring of ports and is held up above them, and the 
flow of steam is below the valve and towards the cylinder. The 
valve-shell or casing is heated by the passing steam, but the 
valve is out of the way and becoming cold, relatively. The 
point of cut-off arrives and the cold valve is suddenly dropped 
across the port and steam shut off. The steam shut back by 
the valve is deprived of its superheat by the cold valve, and the 
under face or interior of the valve is now exposed to the falling 
temperature of the expanding and exhausting steam in the 
cylinder and is quite free to be lifted again to admit the next 
charge. 

Superheated steam is of so small a proportion of the weight 
of the parts of the cylinder that the little that is trapped 
behind the steam-inlet valve soon drops in temperature and 
probably the valve is maintained little or not at all above 
saturation temperature, for it is snatched away from a quiescent 
surrounding of probably saturated steam, kept out of the flow 
of superheated steam until necessary to drop it quickly across 
the flowing stream, and the flowing stream cools as soon as it 
is brought to rest. Such, approximately, are the working 
conditions and, in any valve design, such must be the points 
attended to in order that surfaces may not rub together when 
in contact with superheated steam. Needless to say, the 



BEHAVIOR OF ENGINES WITH SUPERHEATED STEAM 69 

piston-valve has no load on it and is not to be compared with 
the slide-valve or the Corliss valve, both of which have rubbing 
surfaces exposed to steam loading. 

While the Corliss valve will work with superheat of about 
500° F., the slide-valve will fail at temperatures very much 
less, depending on the conditions, but the drop-valve will, it 
is claimed, work at 600° or even at 650° F. temperatures, at 
which it is claimed some Continental engines work satisfactorily 
and economically. In Great Britain the tendency is to use 
lower temperatures. Where high-temperature superheat has 
been tried in individual cases it has not been admitted to the 
high-pressure cylinder, but has been passed through the inter- 
mediate re-heater, thus losing the upper one or two hundred 
degrees of heat and transferring them to the expanded and 
now wet steam exhausted from the high-pressure cylinder. 
In theory of course the full temperature employed in the first 
cylinder should give the best economy, but practical considera- 
tions always carry the greatest weight as theoretical considerations 
begin to approach their limits. Some engineers will decline 
under any circumstances to consider superheat beyond 100° F., 
and no doubt this degree of superheating does give a large 
economy, while further additions give economies less and less 
greater than their cost in fuel and money and worry. Very 
high superheat is no doubt theoretically of value. It may be 
worked with care. But it is somewhat academic and its virtues 
are extolled usually by men whose experience does not extend 
far into the practical. It is better and sounder policy to be 
content with moderate temperatures, increasing if the experience 
acquired warrants such an increase. Especially with uncon- 
trolled superheaters is it important to maintain moderate 
average temperatures, since at any time an unusual maximum 
may occur. 

Since there is always a maximum temperature that must 



70 SUPERHEAT AND SUPERHEATERS 

not be exceeded, the controlled superheater will always supply 
steam of higher average temperature than will the uncontrolled 
superheater, for the average will always be nearer to the maxi- 
mum. Thus in Fig. 10 if AB is the maximum allowable tem- 
perature and CD is the ordinary minimum temperature of 
an uncontrolled superheater, the mean temperature will be 

A n 



/> 

A7 * 



O- — £ 

Fig. 10 

EF for the uncontrolled apparatus. The minimum ordinary 
temperature of controlled superheat will be MN and there- 
fore the average will be OP. An uncontrolled apparatus is 
in fact at a great disadvantage in regard to mean temperature, 
for its maximum cannot be allowed higher than the maximum 
of the controlled superheater, and this necessarily reduces the 
average temperature attained. 






CHAPTER IX 

CONTROLLABLE SUPERHEATERS 

Owing to the variable states of a furnace and the wide range 
of temperature of the flue-gases and the small specific heat 
of steam and of the steel from which pipes are made, an ordinary 
small-tube superheater cannot be fully controlled as regards 
the temperature of its output. The. possible control has been 
referred to under the heading of Small-tube Apparatus. 

But in high-class working the superheater should if possible 
be economically controllable. 

Control may be attempted in various ways, especially in 
•regard to the maximum ranges of temperature: 

1. The steam when becoming too hot may have some auto- 
matic injection of a fine stream of water into the superheater 
pipes, the evaporation of which water will bring down the 
temperature of the superheated steam. 

2. When the temperature is becoming excessive the hot 
gases may be automatically sent away by a by-pass flue and 
damper. 

3. A common and wasteful device is to open a damper 
which drenches the flues with cold air. This mexhod is wasteful 
of heat, but is much employed where the small-tube superheater 
is of a type to burn out readily. 

4. A superheater has naturally some heat-inertia effect 

71 



72 SUPERHEAT AND SUPERHEATERS 

due to its own mass, which is equivalent in heat-capacity effect to 
about one ninth of an equal weight of water. This naturally 
suggests heavy pipes and headers, and this effect is enhanced 
in one superheater by the insertion into the superheater tubes 
of cores of cast iron, the additional mass of which adds to the 
heat-inertia effects of the apparatus. 

5. The temperature of superheat has been kept low and 
safe by passing the superheated steam through copper pipes 
in the boiler water-space. Such a system practically takes away 
all superheat and merely leaves the steam with a remnant of 
superheat sufficient to claim as evidence that the steam is dry. 

6. Finally, a genuine temperature control may be secured 
by passing hot water in a constant stream through pipes threaded 
inside the superheater pipes. The flow of water is urged by 
an inspirator worked by the superheated steam itself, so that 
as the temperature rises the energy of the steam is increased 
and the flow of hot water is rendered more rapid and more 
heat is taken from the steam, which is thus tempered to any 
degree desired. 

Each of the foregoing methods of control may now be con- 
sidered in more detail and seriatim, but first the general loca- 
tion of a superheater may be studied. 

Superheaters are still and generally in the past have been 
placed in some part of the flues of a boiler past which flow hot 
gases that have already passed by or over a considerable area 
of the boiler heating-surface. The gases have thus become 
considerably chilled, and in all superheaters where there is no 
control of any sort the location should be at some point where 
the gases are not more than 1000° F. in temperature. Iron 
heated to 1000° F. is already red-hot, so that a superheater 
must not stand idle very much at such a temperature, for 
iron when red-hot decomposes water and is oxidized by the 
oxygen produced. Hence the provision of means of flooding 



CONTROLLABLE SUPERHEATERS 73 

superheaters with water in order that when not acting as super- 
heaters they may act as an integrant part of the boiler heating- 
surface. The limitation of position thus enforced often prevents 
a very great efficiency of the superheater, for it is frequently 
not possible to get from it steam superheated by more than 50° F. 
In order to get ample superheat with an apparatus of moderate 
size some superheaters have been placed for safety in flues where 
the temperature is moderate, and they have been made of cast- 
iron gilled pipes, the trouble with which, when the gills are 
deep, has proved to be that, while efficient when clean, the 
gills quickly become choked with flue-dust and the capacity 
rapidly falls off. It therefore seems generally desirable to 
employ plain circular tubes and preferably placed in a vertical 
position. Nevertheless cast-iron superheaters have been con- 
siderably employed in Alsace and have perhaps fallen out of 
their one-time use more because of the rise of modern pressures 
than because of any other unsuitability of cast iron. It is 
difficult without control to exercise much choice over the 
placing of a superheater. In the case of the Lancashire type 
boiler the end of the boiler in the back downtake presents 
itself as a suitable place, and it is perhaps a trifle too hot for 
the small-tube superheater and perhaps a trifle too cool for a 
fully water-controlled apparatus. Uncontrolled superheaters 
must be regarded from two standpoints: 1st, from that of 
the superheater itself, and 2dly, from that of the engine. A 
single superheater will send, to the engine, steam of widely 
different temperatures, and it is obvious that where an engine 
is served from one or two superheaters only it may occasionally 
receive steam at or near the maximum temperature due to best 
furnace conditions, and there is a certain maximum beyond 
which it is dangerous to expose an engine, such temperature 
being lowest with valves of the sliding or Corliss type possessing 
rubbing contacts. Now it is certain that with a large number 



74 SUPERHEAT AND SUPERHEATERS 

of superheaters each of which gives a correct mean temperature 
the mixture of the steam from the whole battery will not differ 
all the time from the steady mean temperature desired, albeit 
in turn every superheater may be destructively overheated. 
Thus in large installations the general effect in the engine-room 
may be good, but there will be apt to be frequent destruction 
of superheaters. 

It becomes a commercial question as to the desirableness 
of obtaining a sufficient superheat by allowing all the super- 
heaters in turn to become overheated. This is what is said to 
happen in Germany, where they find the economy to be greater 
than the loss in burned pipes. 

Wethered, the American engineer already referred to, 
endeavored to regulate superheat by mixing with the highly 
heated steam passing through the superheater, ordinary satu- 
rated steam direct from the boiler. 

The regulation system No. 1 is somewhat on these lines. 
Water and superheated steam cannot coexist in intimate con- 
tact. Either the steam will become saturated or the water will 
be absorbed. In any case the temperature of the superheated 
steam will be promptly reduced by the rendering latent of so 
much of its heat in evaporating the water. Wherefore, by 
means of a thermostat placed in the path of the steam as this 
leaves the superheater, a fine jet of water may be caused to be 
sprayed into the superheater about midway of its length or at 
some other point, such jet of hot water being admitted through 
a controlled needle valve. 

By method No. 2 the temperature of the superheated steam 
should be made automatically to by-pass some of the hot gases 
so as to reduce the amount of heat passing by the superheater. 
Ordinarily such by-pass dampers are merely worked by hand 
when the temperature of the steam is seen to be ranging towards 
the allowable limits, but this is crude and unsatisfactory, and 



CONTROLLABLE SUPERHEATERS 75 

an automatic control is desirable, for even at its best the control 
through by-pass dampers is not at all perfect. 

System No. 3 is named only to condemn. It is beyond 
good practice to admit cold air to mix with the hot gases in 
order to reduce the superheat. It means that the superheater 
is made to give an excessive action and that this is moderated 
by heating up so much cold air to be thrown away to the 
chimney with all the heat it has taken up. Nothing could be 
worse or less scientific in practice than this air-drenching method 
of regulating superheat temperatures. Yet engineers can 
be found who defend it, who even argue that it cannot possibly 
be wasteful. Evidently they overlook the fact that all gases 
go to waste at a temperature considerably above the initial 
temperature of the atmsophere; that heat-absorbing surfaces 
cannot act efficiently on large volumes of moderately heated 
gas as they can upon smaller volumes containing the same 
total heat. Yet air-drenching simply dilutes the furnace-gases 
from two to four times and turns to waste so many times more 
heat which cannot be taken up by the subsequent surface of 
feed-heaters, for there is now too little head of temperature to 
promote an efficient flow of heat to the feed-water. Air- 
drenching should therefore not be employed, but it may be 
found in certain types of apparatus which must be protected 
from destruction by the overactivity of the furnace in the 
case of independently fired apparatus, and it would appear to 
be merely a concomitant evil of general poverty of design. 
The air is admitted through openings of large size more or less 
concealed. 

In method No. 4 we have a development of the mass or 
inertia effect which is a property of all superheaters. A thick- 
ness of metal lies between the hot gases and the steam, and 
this metal has a specific heat about one fifth of steam and a 
mass many times greater than the steam-flow per second past it. 



76 



SUPERHEAT AND SUPERHEATERS 



Obviously such metal must go quite a considerable way in 
modulating the sudden changes of temperature that the hot 
gases would give to the steam. In Fig. 11, if the peaked line 
is a curve of waste-gas temperatures, the mass effect of the 
tube metal may cause the range of temperature of the steam 
to vary only along the full curve. These curves are illustrative 
and diagrammatic only and in no sense observationally or 
experimentally exact. The maximum range of steam tempera- 
ture will at least be less than that of the gases, and this product 







Fig. 11 

of mass inertia will be the greater according as the mass of 
the superheater is greater. 

To secure a considerable mass effect, Cruse, when he does 
not employ full water-control, places cast-iron cores of cruci- 
form or combined cruciform and circular section within the 
superheater tubes, and these cores absorb heat from the passing 
steam during periods of rising temperature and discharge it 
back to the steam when the gases are flowing colder. The 
controlling effect is thus very materially increased and the 
superheater pipes which first encounter the hot gases are kept 
cooler. These control or heat-inertia cores are seen in Fig. 12. 

In some cases the cast-iron cores are arranged to act after 
the manner of the Field tube of a boiler, the steam passing 
down the core-tube and returning between the two tubes, but 



CONTROLLABLE SUPERHEATERS 



77 



in other cases the steam is passed only in one direction, the 
inner pipes of cast iron being merely so much mass for accu- 
mulating or giving forth heat. In really high-class control 




12 loop size 



Larju 



If 



Fig. 12. — Tubular Heat-accumulator Superheaters 

the mass action, however, is not sufficient to give steady tem- 
perature. 

Some years ago one McPhail endeavored to secure a control 
by means of water. First he superheated the steam, and then 
passed it through a series of copper pipes laid just below the 
water-level in the boiler. He then passed the steam through a 
second series of superheating pipes, and again through copper 
tubes in the boiler. The net result was that he obtained only 



78 



SUPERHEAT AND SUPERHEATERS 



about 9° of superheat, and this would usually all disappear 
before the steam reached the engine. He did, however, effect 
two things. First he effectively dried the steam and delivered 
it cooled from a high temperature down to 9° of superheat, and 
he probably obtained a really better and drier steam than if 
he had simply heated up wet steam to 9° of superheat. It 




Fig. 13. — Water-controlled 32-pipe Superheater 



may only be opinionative, but there are those who consider 
that there is more heat in steam at a given temperature, if it 
has previously been hotter than the given temperature, than 
if it had been colder. The idea is that the molecular structure 
of the steam-gas is more perfect. Secondly, he appeared to 
add greatly to the capacity of the boiler to make steam, and 
it was even claimed that water-tube boilers, ordinarily giving 



CONTROLLABLE SUPERHEATERS 



79 



one unit of wet steam, now gave two and even three units of 
thermometrically dry steam. 

In brief the control aimed at appeared rather to be a means 




Fig. 14. — Independently Fired Water-controlled Superheater 

of promoting evaporation than of securing superheat, for the 
system employed controlled it altogether — out of existence 
nearly. 

The method of control by water was, however, not yet 



SUPERHEAT AND SUPERHEATERS 




CONTROLLABLE SUPERHEATERS 



81 



quite exhausted. It was revived by H. Cruse in an entirely 
novel manner, though this method of control has involved 
somewhat more costly apparatus than suffices for ordinary 
uncontrolled apparatus. 

In the Cruse controllable superheater, assuming a heavy 
6-inch superheating tube in series of sixteen pipes of, say, 
8 feet in length each, the pipes are joined end to end by means 




Fig. 16. — The Cruse Superheater. View of Header of Superheater Pipes 
and of Internal Water-control Pipe 

of header-boxes with suitable pressed-steel dished covers of 
a sufficient capacity to contain the junction-pieces of internal 
2-inch copper pipes, also united in series through the super- 
heater pipes. In later patterns the outer steel pipes are 
4 inches diameter and the inner water-control pipes are 2 
inches, the steam passing between the two. (Figs. 13, 14, 15, 
16.) 

In the Foster superheater of the Steam Specialties Co. of 
New York the superheater tubes are protected by gills of cast 
iron which are shrunk tightly on the tubes. These gills not 



82 



SUPERHEAT AND SUPERHEATERS 



only protect the tube but give an inertia-controlling effect, for 
they require time to become hotter when exposed to a flow 
of hotter gases, and they are slow to cool when the gases begin 
to flow colder. They temper the heat fluctuations, as shown 
already in Fig. 11, and their protective effect on the pipe is 
good (Figs. 17a, b, c). 




Dctail or Retui 



Fig. 17a. — Portion of Foster Superheater, showing Ends of Elements Con- 
nected by Return Header. The elements consist of seamless drawn 
steel tubing protected by cast-iron rings shrunk on. Inner tubes are 
closed to steam which is thus forced through thin annular spaces and 
rapidly superheated. 

The salient point of the Cruse system is of course the peculiar 
method of water control, which not only regulates the tempera- 
ture within narrow limits, but preserves the superheater tubes 
from injury by excessive gas temperature, or at times when the 
volume of steam passing through is greatly reduced, or when 
steam is being raised after lighting up. The controlling element 



CONTROLLABLE SUPERHEATERS 



83 



consists of a stream of water circulating from one end to the 
other of the superheater through steel or copper pipes inside the 
steel superheater tubes. As may be seen from the illustrations, 
the weldless steel superheater tubes are of large bore, 6 inches 
in diameter, assembled to form semi-independent elements, 




176. — Newer Type of Foster Superheater 



each element containing from six to sixteen pipes, and the 
number of elements varying according to the importance of 
the apparatus. The elements are built to form spirals, and this 
gives to the steam a fair length of travel in the superheater. 
The internal water or controlling pipes are of solid drawn copper 
or steel and follow the form and course of the steam superheating 
system. In the flue-fired superheater, as constructed to 
operate with the Lancashire boiler, the steam enters the super- 



84 



SUPERHEAT AND SUPERHEATERS 



heater at the back and travels zigzag, in counter-current to 
the heating-gases, to the front. The controlling water travels 
concurrently with the steam from back to front, and is taken, 
first, from the boiler water-space; secondly, from the economizers; 
thirdly, from the hot-well or cold main; and after traversing 
the various elements of the apparatus, is collected into one 
stream, and enters, or re-enters, the water-space of the boiler, 
alway below low-water level. The factors determining the 
use and proportion of any one or of all these different waters 
are the heaviness of the firing, the weight of steam to be passed, 







Fig. 17c. — Foster Superheater. 



and especially the maximum temperature of superheat required 
• to be added to the steam. Where high temperatures are wanted 
the circulating water taken from and returned to the water- 
space of the boiler is used alone, and the flow is regulated or 
governed by a steam-jet connected to the superheated-steam 
collector. The steam-jet is initially set as may be required to 
give such flow of water as will allow of a given average tempera- 
ture of superheat, after which the governing effect becomes 
automatic, balancing the water-flow by means of the greater 
or less value of heat or kinetic energy liberated from the super- 
heated steam by contact with the water. In the independently 
fired apparatus the superheater proper is built on lines similar 
to the flue-fired type, with this essential difference, that the 
pipes are placed horizontally, and the steam and water travel 



CONTROLLABLE SUPERHEATERS 85 

from the collectors and distributors at the bottom to the col- 
lector and drum at the top of the apparatus, in counter-cur- 
rent to the heating-gases. 

The use of the water from the boiler, or from the economizer, 
or from both in any proportion enables the control to be very 
thoroughly regulated. 



CHAPTER X 



SUPERHEATING AS AN ELEMENT IN STEAM GENERATION ON 
THE STAGE PRINCIPLE 



Just as the use of steam has come to be by steps or stages 
by means of the various types of compound engines, so also 
is it gradually being realized that the manufacture of steam 
must for economical reasons be carried out in stages. The 
first stage is that of heating the feed-water b}' heat that would 
otherwise be wasted. The second stage, if required by the 
insufficiency of the first, consists in adding heat to the feed- 
water until it becomes as hot as the boiler. The third stage is 
the addition of the latent heat of evaporation to the now fully 
heated feed-water whereby it is converted into steam; while 
the fourth and last stage is that of superheating, the pressure 
of the steam remaining constant while a further supply of heat 
is put into it, raising its temperature above the point of satura- 
tion and enlarging its volume, producing in fact a true steam- 
gas which does not liquefy if cooled so long as the cooling does 
not extend beyond the point of saturation temperature, or to 
the abstraction of latent heat, whereupon condensation would 
ensue. 

In every-day practice, in order to economize heat to the 
fullest possible extent the feed-water is taken first from the 
condenser at or about 100° F. If after it leaves the condenser 



SUPERHEATING ON THE STAGE PRINCIPLE 87 

it is necessary to purify the feed-water of oil, whereby it becomes 
cold, there is no reason why it should not be re-heated in tubes 
forming an advanced portion of the surface condenser and 
then taking heat afresh from the exhaust. The feed-water, 
now as hot as it can be made by exhaust-steam, then passes 
to the economizer, wherein it is heated by the waste gases from 
the boilers. The extent to which feed may thus be raised in 
temperature must be a matter of the area of the heating-surfaces 
of the boiler and of the economizer relative to the fuel burned. 
As a rule the feed leaves the economizer at a temperature 
less than that of the boiler. It is therefore now to be subjected 
to a further heating. Either it must pass through a live-steam 
heater, in which it is directly exposed to live steam from the 
boiler, or it may be heated to full boiler temperature in the 
water-control pipes of controllable superheaters where these are 
employed (as to which see Chap. IX). Suffice that once it has 
attained boiler temperature it may be turned into the boiler, 
there to receive the latent-heat supply necessary to convert it 
into steam. The heat of the furnace is directly converted into 
latent heat of steam; there is to be no heating of water inside 
the water-space of the boiler. The boiler will perform its duty 
more efficiently when this duty consists in one stage only of 
the process of steam production, namely, the evaporative duty. 
The newly formed steam is wet. It is to be taken by way of 
an anti-priming pipe to the superheater, and there the finishing 
touches of drying and superheat are carried out. Were it not 
that our materials of construction will not bear exposure to the 
furnace temperature, and that steam above 500° to 700° can- 
not be dealt in by the steam-engine, the superheater should 
theoretically be placed in the hottest part of the furnace. As 
it is, a compromise has to be made, and the addition of the 
summit of temperature to the steam has to be effected by 
aid of gases already deprived of some heat by the boiler. 



88 SUPERHEAT AND SUPERHEATERS 

This is not as things should be, but it is the best that materials 
allow us to carry out. 

Superheating is thus one in the series of stages into which 
the best modern practice divides the process of steam production. 
The attempt to carry out in the boiler the dual functions of 
feed-water heating and steam generation is and always has been 
a failure. We do not here speak of the heating of feed-water in 
the steam-space of a boiler. This is correct enough if carried 
out thoroughly, so that the water actually drops at full boiler 
temperature into the water-space. There must be no chilling 
of the water in a boiler. A boiler can only produce steam 
somewhat incomplete. Such steam is very readily condensed 
and superheating is necessary to render the steam perfect. 
Freshly made or saturated steam no doubt contains many im- 
perfectly formed molecules, and it is necessary that these should 
be rendered perfect even if not further energized by liberal 
superheating. 



CHAPTER XI 

SUPERHEATERS 

There are two main types of superheaters: (1) those which 
are placed in the flues of a steam-boiler, and (2) those which 
are placed in the flues of an independent fire. Of each class 
there are practically but two types or classes: (a) The small- 
tube type, without special control of temperature, and (b) the 
large-tube type, in which special means of controlling tempera- 
ture are adopted. 

The flue-heated superheater gets its heat from gases which 
have already given up a portion of their heat to a boiler. They 
are therefore found placed in such a position that the gases 
passing through them have been deprived of much of their 
heat. If, for example, the furnace temperature of a boiler was 
3000° F. and it was considered that the superheater could not 
be exposed to gases much above 1000° F., then there must be 
so much of the heating-surface of the boiler placed between the 
furnace and the superheater as will serve to abstract from the 
gases the 2000° of temperature. This is where the difficulty of 
arranging superheaters safely comes in. In the Lancashire or 
return-tube shell boiler the back downtake is the obvious loca- 
tion for the superheater : the gases have been deprived of much 
of their heat in travelling through the internal flues, and they 
are not far from the temperature at which it is safe to work a 



90 SUPERHEAT AND SUPERHEATERS 

superheater of the small- tube variety. With water-tube boilers 
there is apt to be greater difficulty, for the location cannot be 
before the first bank of tubes, and these usually do so large a 
share of the total work of a boiler that the gases which have 
passed this first bank of tubes are usually too cold to give effec- 
tive superheat. If fired sufficiently to give satisfactory super- 
heat, the boiler efficiency will be low, and if worked at an 
economically efficient rate, the degree of superheat will be insuf- 
ficient. 

Marine boilers are not very suitable for flue-fired superheater 
location, for they are too short sufficiently to reduce the gas 
temperature to a safe point, and after the gases have made the 
pass of the tubes they will be too cold. For all such boilers, 
therefore, it is more desirable to employ superheaters of the 
separately fired order, which if properly constructed will add to 
the steam and water capacity of the boiler plant, usually some- 
what small where water-tube boilers are employed. 

Almost all superheaters are made up of a series of coils 
or hoops or banks of pipes through which steam is passed, and 
they are immersed in the stream of hot gas in the boiler- flues. 

Taking the small-tube type first, these are built up of loops 
of small tubes through which it is too often the custom to hurry 
the steam, which at a high velocity more readily "brushes" 
the heat off the pipes. They are apt to become choked with 
scale carried over in the water of priming. Hence the method 
of attachment of the tubes in the example illustrated, the 
superheating tubes rising from the upper side of the header- 
box so as not to carry off any water from the entrance-header 
which might find its way there as priming. It is always diffi- 
cult to control the superheat given by small- tube superheaters. 
Sometimes with good fires the temperature becomes danger- 
ously high. At other times it falls very low. 



SUPERHEATERS 91 

Where there are several boilers, each with its superheater 
delivering steam to one main steam-pipe, the average tempera- 
ture of steam at the engine stop-valves will not perhaps vary 
much, for the extreme conditions of temperature do not occur 
simultaneously in all boilers. The several streams unite to 
give a stream of average temperature and the engine is thus 
more or less safeguarded against overhot steam. In some 
small-tube superheaters the deliberate intention is to have a 
small total area of passage through the tubes, so as to raise 
the velocity of the passing steam, though at the cost of pressure, 
but this method has the disadvantage of throttling down the 
steam-pressure in order to assist the frictional heat-absorbing 
effect upon the tubes, for, as above stated, rapidity of flow 
induces a picking up of heat by the steam from the tube interior 
surfaces. Cases are known where as/ much as 15 pounds 
pressure per square inch has thus been throttled out of the 
steam. 

AYethered tried superheating a portion only of the steam, 
which he mixed overheated with the remainder of saturated 
steam. This was called combined steam, but it was really 
only the mixture which is every day got from any bank of 
superheaters the average output temperature of which is the 
combined temperature of the whole of the superheaters in the 
bank. But the chief trouble with small-tube apparatus has 
been to protect them from burning when the flow of steam 
through them is for any reason stopped. In some cases the 
superheater is directly placed in free communication with the 
boiler water-space by means of valves which are or should be 
suitably interlocked. The superheater is made a part of the 
heating surface of the boiler, but when it is again wanted to 
act as a superheater all its water contents must be blown 
out hot and more or less heat is thus wasted. Used as an 
evaporator also, there is a tendency to fur up the interior of 



92 SUPERHEAT AND SUPERHEATERS 

the superheater tubes. In other cases the superheater has been 
placed in a chamber when room has been available which may 
be cut off from the path of the hot gases by a short-circuiting 
damper. When not passing steam the damper is opened to 
by-pass the gases directly instead of by way of the superheater. 

Placed as they arc, directly in the path of hot gas, super- 
heaters are exposed to severe conditions, for long periods to the 
influence of a hot fire, then to much milder temperature, while 
the rapid changes of temperature cause expansion and con- 
traction of the tubes and deterioration of their substance. A 
boiler-furnace crown is only some 50° to 150° hotter than 
the water in contact with it, according to the cleanliness of 
the plate surface and the conditions of velocity of the gas and 
of the water on its opposite faces. 

Now the mean temperature of a plate is found by Bryant to 
be half the sum of the temperatures of the steam and the 
gas on its opposite sides. This when the steam moves fast. 
When the flow is slow the tube may become hotter. Obviously, 
then, a superheater pipe may become very highly heated. Super- 
heaters, therefore, should not be regarded as cheap accessories 
to a steam-boiler to be constructed as mere ephemeral appara- 
tus. 

Correctly to approach the question of superheat, efficiency 
should come first and is to be secured with strength, safety, 
and durability. The best and most trustworthy materials 
must alone be employed and put together with the same skill 
and care "as the best class of boiler-making. Thus no cheap 
apparatus can be satisfactory, for the gases must be at least 
1000° F. for efficiency, or even 1200°, or, say, 500° to 600° 
above the steam temperature. The small-tube uncontrolled 
superheaters cost considerably less than the controlled super- 
heaters and are of course far less durable. In every instance 
the engineer responsible must take up this question of relative 






SUPERHEATERS 93 

cost and durability for his own particular case, endeavoring to 
ascertain the life of the small-tube type under conditions 
similar to his own, and comparing the lower first cost and higher 
renewal and maintenance charges with the heavier capital 
charges and low repairs and maintenance of a fully controlled 
superheater of the fully water-controlled type. 

Many different superheaters have been tried in Germany, 
and the consensus of opinion is, the author believes, all in 
favor of continuing the practice of superheating in spite of the 
constant annoyance and trouble of burned-out tubes and 
repairs. The Germans, however, have not yet realized the 
value of the water-control system, being, like so many English 
engineers, taken by cheapness of first cost and also, perhaps, 
not realizing the peculiar action of the water-control principle, 
as to which more anon. 

But whether of control or small-tube variety, the tubing 
must be of the best quality of weldless steel or iron, and the 
construction must be first class. Indeed already the small- 
tube apparatus is in many cases being constructed upon designs 
much superior to those which prevailed only a few years ago. 

Any superheater, especially if of large size, ought to be con- 
structed of several elements, so put together that any one can 
readily be detached or cut out, leaving the remainder to work 
without it. The passages for the hot gases should be of suffi- 
ciently small area to compel the gases to sweep over every 
part of the superheater surface, yet without choking the draft 
by undue throttling. The heat transmitted through each unit 
area of tube surface will depend upon the head of temperature 
acting to produce flow, increasing as this increases. 

The direction of flow of steam should be generally contrary 
to that of the gases, so that the steam will flow from the rear- 
most coils in the direction of the coils nearest the boiler, the 
hottest gas impinging upon the hottest steam-pipes. As 



94 SUPERHEAT AND SUPERHEATERS 

steam travels through the tubes it should be mixed up in the 
header before entering the next tube. 

Since steam expands when superheated, this must be allowed 
for in the area of the tubes unless wiredrawing is to occur, and 
this area through the superheater tubes should not usually 
be less than 1| times that of the main boiler steam-pipe for 
pressures above 130 pounds and a maximum of 250° of added 
superheat. For low pressures the superheater may have a 
passageway 1*33 to 1*50 times that of the steam-pipe. 

Needless to say, since the duty of a superheater is to add 
temperature to the steam, it ought not to be called on to act 
as an evaporator. It ought to have a dry-steam supply, though 
many superheaters are purposely fed from boilers made to prime 
in order to preserve the tubes. 

About 700° F. should be a maximum temperature for super- 
heated steam, having due regard to the strength of mild steel 
at such temperatures. But no lubrication-oil can exist at 650° 
or retain any viscosity, nor can any engine take steam safely 
at 700° F. 

Cast iron is quite unfitted as a material for superheaters, 
though it may be still employed for some of the cheaper sorts 
of apparatus which are little else but steam-driers. It is also 
legitimately to be employed for the internal cores of the accu- 
mulator or heat-inertia type of superheater, or for the gills 
shrunk upon the tubes of the Foster superheater. 

It is for the holding of high-pressure steam that cast iron 
is not suitable for use as superheater pipes, though it is still 
employed to some extent on the continent of Europe, but only, 
the author believes, for moderate degrees of superheat and 
exposed to gases of moderately high temperature only. 









CHAPTER XII 
FEED-WATER HEATING 

Especially when feed-water can be heated by heat that 
would otherwise be wasted great advantage can be obtained 
by heating feed-water. 

To find the economy due to feed-heating by waste gases 
from a boiler, as by means of the economizer or by exhaust- 
steam, as in a coil, the following formula will give approxi- 
mately correct results: 

(Temperature of economizer outlet) — (Temp, of inlet to economizer) 
100 X ( Total heat of steam from 0° F. | VZ * ■ , , , '. \ 

( at the boiler-pressure \ ~ \ Tem P' of inlet to economizer; 

= percentage of gain. 

In addition to the mere gain of heat otherwise lost there 
is a gain in the efficiency of a boiler fed with hot feed, especially 
if fed with water at the boiler temperature, so that the sole 
duty of the boiler is to evaporate water and not to raise its 
temperature. Thus the water fed to a steam-boiler, if not 
already heated to the temperature of evaporation, ought to be 
submitted in a separate vessel to the action of the boiler-steam 
in order that no water may enter the boiler at a temperature 
lower than that of the boiler. The exact reason for the better 
working of a boiler thus fed is not known, but undoubtedly 
there is a better transfer of heat across the plates to hot water 
than to cold water. The reason seems to be in some way 

95 



96 SUPERHEAT AND SUPERHEATERS 

connected with the direct vaporization of the water, all added 
heat becoming latent and none of it being first expended on 
heating the water. 

In the Cruse controllable superheater the final raising up of 
the feed-water to boiler temperature is carried out by causing 
the feed-water to enter the boiler by way of the controlling 
inner tubes of the superheater, which tubes are thus compelled 
to fulfil the double duty of controlling the temperature of 
superheat to the desired point and of adding the heat to the 
feed-water which it lacked from evaporation temperature. 

Though superheating may in al] its methods be combined 
with some system of fully heated feed-water, the superheater 
of Cruse is put forward as rendering such full heating an integrant 
part of the process of steam-raising in conjunction with super- 
heating. This view is of course an ex parte statement of the 
experts of the Cruse system, but is nevertheless entitled to 
consideration in view of the fact that the best steam-engineers 
recognize the soundness of the claims. 

An expert on steam says : 

"The Cruse system of superheating claims to differ entirely 
in principle from an}' other, and also in design and construction. 
It embodies a controlling device which renders it as safe and 
efficient under proper and reasonable conditions of care and 
attention as any water-tube boiler. 

"In giving his reasons for the creation of this novel type 
of superheater, the inventor of the Cruse system points out 
that in the small-tube superheater the tubes are liable to 
become overheated when the temperature of the gases is high 
and when the draft of steam flowing through the pipes becomes 
restricted owing to the engine being stopped for a short time, 
the normal flow of steam thus being arrested. The tubes, having 
no heat-conveying medium to relieve them, will become red- 
hot and will soon show signs of decay. When the engine is 



FEED-WATER HEATING 97 

again started, unless the first steam drawn through the super- 
heater is blown to waste, or other precautions be taken, the 
steam will reach the engine at a dangerously high tempera- 
ture, something is apt to seize and a serious smash may follow. 
In order to prevent these small tubes from suffering too rapid 
destruction, the aggregate steamway or tube area is purposely 
made small so as to increase the velocity of the steam and its 
brushing action on the pipes. This means a serious drop of 
pressure which has been found to amount to 12, 15, and even 
25 pounds per square inch. Loss of pressure, even to the 
extent of the first of these figures, deprives the engine of much 
of the economy which it gains from superheat. 

" Obviously, therefore, the desiderata of a really good 
superheater are, first, such an area through the pipes as will 
insure that the loss of pressure of the steam shall not exceed 
1 to 2 pounds; secondly, a method of preservation which, while 
allowing for high superheat, will insure that this shall not be 
accompanied by overheated tubes and will provide a heat-con- 
veying medium to relieve the pipe metals when the flow of 
steam is arrested; and thirdly, an effective power of controlling 
the temperature of superheat within narrow limits. One cause 
of the lack of control of small-tube superheaters, it may here be 
remarked, arises from the small mass of the tubes, which 
causes them to follow the fluctuations of the gas temperatures 
too closely, and thus causes the steam also to fluctuate widely in 
temperature. 

"In the Cruse system the superheater pipes are made large 
and heavy, their dimensions being often 6 inches diameter by 
■A- inch thick. They are so grouped as to afford a steamway 
from 25 to 50 per cent in excess of the area of the boiler steam- 
pipe. Thus in a 32-pipe superheater placed behind a large 
Lancashire boiler the superheater consists of four sections of 
eight pipes each. That is to say, the steam from the boiler 



98 SUPERHEAT AND SUPERHEATERS 

flows through the superheater in four "parallel" streams, which 
again unite beyond the superheater into a single stream at the 
main steam-pipe. 

"To some extent the heavy mass of the tubes serves as a 
store of heat. A sudden accession of temperature in the gases 
does not produce a rapid change of temperature in the pipes, 
nor does a sudden fall chill the pipes so rapidly as if they were 
of the thin small type. But this form of control is merely 
incidental. The designed control is effected by water. A 
water branch is taken from the boiler at low-water level and 
water is drawn out at this branch and forced through 2-inch 
copper or steel pipes which traverse the superheater pipes from 
end to end and return to the boiler. Thus each end of this 
pipe is exposed to boiler-pressure, and to force water through 
it only demands energy sufficient to overcome the friction, 
which is not great in a solid drawn smooth pipe. To produce 
the flow the water is passed through an inspirator or aspirator 
fed with superheated steam. When superheated steam touches 
water it at once becomes saturated, and, at usual temperatures 
of ' superheat, it loses, say, 20 per cent of its volume. This 
reduction of volume is, like the condensation in an ordinary 
injector, the source of energy, and serves to propel the water 
through the inner tubes at a considerable velocity. Should 
the gases become hotter and the steam temperature rise, the 
steam volume increases and the action of the aspirator is 
correspondingly intensified. More water flows and picks up 
more heat from the surrounding steam. In this way the con- 
trol exercised by the water columns is automatic, the result 
being that the temperature of superheat varies between narrow 
limits and the danger-point is never reached. 

"In adding 200° F. of temperature to steam, and assuming 
the mean specific heat to be 0*55, each pound of superheated 
steam will absorb 110 B.T.U. Saturated steam produced from 



FEED-WATER HEATING 99 

water at ordinary temperature requires about 1,100 B.X.U., so 
that the extra heat in superheated steam is just about 10 per 
cent. 

11 Some years ago M. Normand, the French engineer, found 
that the economy of a boiler, and its efficiency for heat utili- 
zation, was improved about 10 to 15 per cent by heating the 
feed-water up to full boiler temperature by means of its own 
steam. While this did but look like paying from one pocket 
to another, experience has since shown that a boiler fed with 
fully heated feed really will perform better and more economically. 
No one has quite satisfactorily explained the matter, but the 
effect obtained appears to be due to the much-improved mobil- 
ity of the water in the boiler, and it is acknowledged that a 
boiler is only properly used when its duty is confined to evapora- 
tion. It should add the latent heat of transformation from 
water into steam. Feed-heating, it may here be remarked, 
should be strictly confined to separate vessels. It is to the 
effect just described, says a technical expert, that must, in his 
opinion, be ascribed the apparent production of superheated 
steam by the Cruse system at a cost little if any greater than the 
cost of the same steam saturated. Another advantage of 
fully heated feed-water is that it enables a boiler to be forced 
heavily without priming. 

"Mr: Cruse modestly claims 20 per cent of increased capacity 
on a basis of from and at 212° F., but some engineers state 
that they are actually getting out of water-tube boilers as much 
as 36,000 pounds of steam by feeding at full boiler temperature, 
where formerly they were getting only 12,000 pounds from the 
same boiler. Without pressing its claims so far as that, it 
may be said that by confining it to evaporative duty only, a 
boiler will safely and without ' priming ' give a much greater 
duty. 



100 SUPERHEAT AND SUPERHEATERS 



Independently Fired Superheaters 



"Short dry-back marine boilers are too hot at the back end 
of the furnace-flue for safe superheat and the gases are too cool 
at the front uptake. Water-tube boilers, if worked at rates 
economical for the boiler, are too cold after the first pass of 
the gases to give satisfactory superheat. Hence have arisen 
the many separately fired superheaters which, when of the 
small- tube type, labor under this difficulty, that they cannot 
safely bear a temperature above 1,200° F. Now a furnace 
has a temperature of 2,500° to 3,000° F., and in these super- 
heaters, to cool the gases to a requisite degree, an excessive 
volume of cold air is admitted and the fuel efficiency for these 
superheaters is reduced to between 32 and 40 per cent. In the 
Cruse system of separately fired superheater the same principle 
of control already described for the flue-fired superheater is 
carried out. One superheater may consist of eight elements 
or spirals of 10 or 12 laps of pipe each. Even the Cruse super- 
heater with its inner protection of control pipes is not supposed 
to bear gases above 1,400° F., though probably it might be 
exposed safely to 1,600° F. Plence, in order economically to 
reduce the furnace-gases from 3,000° or 2,500° F., the whole 
of the feed-water for a battery of boilers is passed through 
water-heating pipes placed between furnace and superheater. 
These water-pipes take out the excess of temperature from 
the gases. They also supply water to pass through the control 
system, and a superheater of a capacity of 50,000 pounds 
of steam superheated 200° or 250° F. per hour will heat 50,000 
pounds of water from the temperature of the economizer 
delivery to the temperature of the boiler. 

" A steam-generating plant is thus made up of four elements: 
first, the economizer which heats the feed from condenser 
temperature to 200° F. or 250° F.; secondly, the foreheater of 






FEED-WATER HEATING 101 

the superheater, or the gas-cooler as ii may be termed, which 
heats the feed from economizer to boiler temperature; thirdly, 
the boiler which adds latent heat only; and fourthly, the super- 
heater to heat the steam beyond boiler temperature. The 
waste gas of the superheater goes to the economizer. 

" Apart from the control of the superheat temperature 
effected by the water-tubes these also serve to preserve the 
superheater tubes from burning. Superheated steam is dia- 
thermanous to radiant heat, and chiefly acquires heat by 
rubbing contact with the hot pipes. When these pipes get 
overhot — too hot for safety— they radiate heat powerfully, 
but this radiated heat merely passes across the steam inside 
the pipe and enters the pipe body radially opposite. As all 
radiation is normal to the radiating surface, the heat must all 
pass through the centre of the pipe. But the water-control 
pipe in the centre of the superheater pipe receives the radiant 
heat of the pipe and carries it to the boiler by means of the 
flowing water. The proportions of the pipes have been fixed 
by frequent trial, until a good all-round balance has been 
obtained, and the sj^stem of controlled superheat is becoming 
recognized as the only safe and reliable system, as it obviously 
must be, with the water-control. Indeed, the water-control 
inner pipe has been found in practice to allow the superheater 
to be left for many hours in the hot gases when no steam is 
flowing through the tubes except that which is taken up in 
feeding the aspirator. 

"A separately fired superheater of standard type with 
water-tube foreheater has ten elements of superheater pipes, 
each 112 feet long, for a special case where 60,000 pounds of 
steam per hour might be required to be superheated by 300° F., 
or 75,000 pounds through 200° F., or 100,000 pounds only 150° F., 
the same rate of feed-water being additionally heated 130° F., 
120° F., or 100° F., respectively. 



102 SUPERHEAT AND SUPERHEATERS 

"The growth of the steam-turbine has of late caused con- 
siderable interest to be taken in superheating. Superheat is 
recognized as necessary to the economy and safety of the tur- 
bine. But variable temperature introduces an element of 
danger. Very high temperatures, such as may occur occasion- 
ally, are fatal. To avoid these the small-tube superheater 
must be so devised that its maximum shall be safe, and this 
compels a very low mean superheat. To secure even this 
modicum of safety necessitates a throttling or wiredrawing 
superheater; and even 25 pounds loss of pressure is sometimes 
incurred, but generally the loss is nearer 15 pounds. Hence the 
employment, in the superheater described, of larger tubes with 
a long run for the steam. 

"The facts gained by experience have stimulated engineers 
to devise independently fired apparatus, but the gas tempera- 
ture has always had to be tempered by excess of air with the 
result of low efficiency. It is claimed that only by using the 
surplus temperature, as in the Cruse system, can a real and full 
economy be secured, and by just so much as the superheater 
grate-surface serves to add heat to the feed-water, by so much 
it enables the provision of boilers to be reduced. In addition 
to this there is also the Normand effect, which will be secured 
wherever feed-water is fully heated to the temperature of 
evaporation. The Halpin thermal storage system secures this 
Normand effect also, and attention has been quite generally 
turned to this point. 

"Steam-generation has always been far too haphazard a 
process, and now since superheat has come along the evil 
effects of this haphazard system of working have become more 
apparent and there is perhaps to-day a growing desire to place 
the art of steam-generation on a more scientific and satisfactory 
basis." 






CHAPTER XIII 
EXAMPLES OF SUPERHEATERS 

The Foster Superheater 

This is a small-tube superheater on which the tubes are 
protected from the direct action of the hot gases by means of 
gill-rings of cast iron shrunk on as shown in Fig. 17a, which 
gives a partially sectional view of two tube-ends and a header- 
box and shows the internal-studded tube by the agency of which 
the steam is made to travel in a ring-space and to present itself 
in a thin hollow column against the internal surface of the 
superheater tube. 

The tubes are straight and parallel to each other and ex- 
panded into the headers, which are fitted with bolted caps 
as shown. The headers are of hammered steel bored longi- 
tudinally for steam and crosswise for the tube-ends and hand- 
holes. 

This superheater is placed in the back downtake of a Lan- 
cashire boiler, in the space under the drum of a Babcock or 
similar boiler, Fig. 18, high up against the front tubes in the 
Stirling boiler, and it is placed vertically or horizontally ac- 
cording to the necessity of a given case, being vertical, for 
example, in the uptake of the return-tube boiler. 

Fig. 19 shows this superheater in a portable form arranged 
as an independently fired apparatus. The gills are not pro- 

103 






104 



SUPERHEAT AND SUPERHEATERS 



tectors only; they serve also as heat-collectors, presenting an 
enlarged surface to the gases as well as giving the heat-inertia 
or controlling effect described in an earlier chapter. 

A superheater similarly controlled by mass effect is Cruse's 
accumulator superheater, Fig. 12. 

This superheater was devised to meet a demand for a low- 
priced alternative to the well-known water-controlled superheater. 




Fig. 18. — Foster Superheater in Edgemore Boiler 
In this boiler the superheater is placed just above the tubes. A portion 
of the hot gases in the first pass acts on the superheater, which is supported 
from overhead beams. The headers are of wrought steel and are connected 
to the boiler steam-space by protected pipes. Doors are provided for access 
to the superheater independently of the boiler. 

It does not possess the same nicety of temperature control, 
nor does it possess the property of increasing the feed-heating 
and evaporative duty and efficiency of the boiler; nevertheless, 
and notwithstanding that it has no metal-preserving water- 
core, by reason of the heavy mass of the outer pipes (4 inches 
outside diameter by i-inch shell), and of the internal cast-iron 



EXAMPLES OF SUPERHEATERS 



105 



vertical-gilled accumulator tubes, this apparatus possesses a 
heat-inertia effect which tends to tone down those violent 




Fig. 19. — Foster Separately Fired SuDerheater (Portable) 

fluctuations of temperature which frequently occur with the 
usual small-tube superheaters in vogue, whether loop or " Field" 
tube type, and flattens out the temperature-curve from a 



106 SUPERHEAT AND SUPERHEATERS 



succession of steep peaks and sudden declines into an easy- 
undulation, having a much higher mean temperature, as in 
Fig. 11. 

The external shells and tubes, being the pressure-bearing 
portions of the superheater, are all of wrought steel of unusual 
thickness and strength. The superheater tubes are of mild 
steel and seamless, 4 inches outside diameter, and J inch thick- 
ness of shell, bent to form loops. Each leg of each loop con- 
tains a vertical-gilled tube which terminates at the beginning 
of the bend. Each internal tube has four vertical gills, which 
divide the area of the steel tube into four outer and one central 
cell for the passag of the steam; each outer cell presents three 
effective heating-surfaces against which the flowing steam 
brushes and licks up heat contained in the metals. 

The superheater tubes are expanded into tube plates of 
rolled steel slabs 1\ or 1 J inches thick, with covers of mild-steel 
boiler-plate f inch thick. The end dishes are provided with 
steam inlet and outlet blocks of mild steel, and the central dish 
with a safety-valve and block. 

The gilled heat-accumulator tubes are of cast iron. They 
are not subjected to pressure, because, being inside the steel 
tube-shells, they are open to the general body of enclosed 
steam, and the pressure on all sides is equal. They are, however, 
made massive to accumulate heat, and for this purpose cast 
iron is convenient and economical. 

Steam from the boiler enters the first or inlet box, A, Fig. 12; 
herein it is distributed amongst the tube-ends opening into 
this compartment. It passes down the five cells formed by 
the gilled internal tube in each leg of the loops covered by 
the dish; at the beginning of the bend, where the accumulator 
tube ceases, the five streams of each loop amalgamate and form 
one stream, which traverses the bend until, meeting the accu- 
mulator tube in the second leg of the loop, it is again broken 



: 



EXAMPLES OF SUPERHEATERS 107 

up into five streams, and thus travels into the central box, B. 
The full flow of steam from the boiler now travels in one body 
across this box to be again broken up into as many streams as 
there are cells in the loops. The steam in the second set of 
loops follows the course already described, and finally reassem- 
bling in the third box, C, it leaves this, superheated, for the 
steam-range. 

The mean temperature of the metals of a superheater tube 
(without the internal core) is generally computed as the mean 
of the temperatures of the heating gases and of the heated 
steam. Heat from the gases is communicated to the winged 
accumulator tubes inside the main tube by diffusion from the 
outer tubes and by radiation through the nearly diathermanous 
steam passing through the channels formed by the inner core- 
pipes. Thus the core-tubes endeavor to acquire the tempera- 
ture of the outer tubes, and since cast iron has a specific heat 
of 0'130 B.T.U. per degree per pound for low temperatures, and 
this figure rises with the temperature, a tube which may weigh 
4.480 pounds in a large superheater, if heated to 500° F. above 
the temperature of saturated steam, will represent a heat 
storage of 350,000 B.T.U. , or enough to add 100° F. of super- 
heat to 2,100 pounds of steam. Thus it is clear that in all super- 
heaters the effect of mass cannot be overlooked. It must 
always be of advantage. Such an accumulator superheater will 
give a temperature of 450° to 750° F. to the steam, which is 
well mixed in the passage through the tubes and the divisions 
cf the inner cores. 

This superheater has therefore points in common with that 
of Foster, but differs in the position of the cast-iron inertia 
metal. 



108 



SUPERHEAT AND SUPERHEATERS 



The Water-controlled Superheater 

The author knows of but one example of this type, viz., 
that of Cruse. Fig. 13 shows the 32-pipe apparatus as supplied 
to the Lancashire boiler, in the back downtake of which the 
superheater is placed. The inner tube, through which water 
from the boiler is circulated more or less mixed with the whole 
or a part of the fresh boiler feed-water, serves two purposes, 
Diagram showing the Radiation of Heat from the Heating Gases 



ST£EL TUBE 




20. — Uncontrolled Superheater Tube 
In this type the heat-rays shoot diametrically through the tube area from 
side to side. Tube metals are overheated, and eroded inside and out, when 
steam is stagnant or circulating in restricted volume. 

viz., the control of the temperature of superheat and the pro- 
tection of the pipes of the superheater, especially when steam 
is not passing through them. 

The preservative effect of the inner water-tubes is due to 
the fact that since heat is radiated normally to the radiating 
surface, all rays of heat from the outer-tube inner surface must 
pass through the tube centre and must therefore be intercepted 
by the water-tube there present. Figs. 20 and 21 show a tube 
radiating heat with and without the "inner water-pipe 



EXAMPLES OF SUPERHEATERS 



109 






In the absence of the pipe of water, Fig. 20, the heat-rays 
shoot diametrically across the tube and enter the opposite 
tube-wall. The tube becomes overheated in consequence if 
filled with stagnant steam only. 

In Fig. 21 the heat-rays are all intercepted by the water- 
tube, and the heat is carried to the boiler by the flowing water; 
the tube is thus prevented from becoming too hot by absorption 
of the radiant heat from its own walls. 
Diagram showing the Radiation of Heat from the Heating Gases 



STEEL TUBE 




Fig. 21. — Controlled Superheater Tube 
In this type the heat rays shoot through the steel tube-shell, through 
the steam-ring, into the water column inside the copper pipe. Excess heat 
is carried away by the circulating water into the boiler, the temperature 
of superheat is controlled, and the outer steam-tube metals are preserved 
from overheat and erosion. 

Where this water-control is employed the water in the inner 
pipe would be rapidly evaporated by the superheat outside 
if not constantly renewed. It is therefore kept in constant 
movement, water from the boiler, as shown in Figs. 13, 14, and 
15, being drawn into the control-pipes and discharged again to 
the boiler. Since both ends of the control-pipe are open to 
the boiler, the water in the pipe is free to move easily in either 
direction. There is therefore placed on the entrance to the 



110 SUPERHEAT AND SUPERHEATERS 

superheater an inspirator or water-propeller. The only duty 
this has to perform is to keep the water in movement through 
the pipes of the control system. The inspirator is worked by 
steam taken from the discharge-pipe of the superheater and 
therefore superheated. When this steam enters the inspirator 
and there mixes with the water it may not condense as does the 
steam in an ordinary cold-water-fed injector. But it will 
shrink in volume, for it will become saturated steam and this 
shrinkage represents work energy. Suffice to say that the prac- 
tice of years shows that the water is put into rapid movement. 

Now when the superheat temperature rises, the shrinkage of 
volume becomes greater and the propelling effect is intensified, 
and the contrary is the case when the superheat falls in tem- 
perature. Thus the flow of water increases or diminishes as 
required, and threatened great variations of temperature are 
self -moderated. This is the system of automatic control 
adopted in this superheater, and it is found to work so well 
that the superheat temperature varies only within a small 
range, and having been once set by regulating the steam-supply 
to the inspirator, the further control is automatic. 

This superheater is made throughout of solid rohed or 
pressed weldless steel of high class. The pipes are of large 
size and diameter, the ends are staved for threading and screwed 
into solid steel headers with cover-boxes of pressed-steel plate. 
While this system has justified itself by working several years 
unburn ed, it is not to material alone that success has been due. 
It is the controlling system that gives the safety, certainty, and 
durability of this apparatus. Within the coils of steam-pipe 
are coils of 2-inch solid drawn copper pipe, through which 
circulates water from the boiler, which is returned to the boiler, 
the propelling agent being superheated steam. 

Let us try to follow out the control action by assuming a 
sudden accession of furnace-heat. The superheat temperature 



EXAMPLES OF SUPERHEATERS 111 

begins to rise. As soon as this occurs the impelling action on 
the control-water system is increased, and the flow of water so 

augmented that the superheat rises only slowly. The contrary 
effect follows if the furnace becomes colder, for less control- 
water then circulates. Further control, if required, is given 
by passing a part of or all the feed-water also through the 
control-pipe with the water from the boiler. Usually the boiler- 
water alone is sufficient, and it re-enters the boiler as water 
and steam. The controlling effect of the inside water column 
is displayed not only on the steam. The inside pipe absorbs heat 
radiated through the steam from the outside; pipes. Thus each 
heated pipe is radiating heat all the time upon a comparatively 
cold inner water-pipe of great absorptive capacity, and the 
outer pipes will bear immersion in gases so much hotter than can 
mere steam-filled pipes. It appears simple, and yet, given 
properly calculated sizes and areas, the controlling influence of 
this inner column of water is sure. By its means the superheater 
may always be left in the hot gases; the water circulation is 
continuous, and the superheater forms a part of the boiler 
whether steam be passing through or not. Indeed, it is claimed 
to add frequently 10 to 15 per cent to boiler efficiency. As a 
fact, of course, some steam always circulates, for the control 
column draws its power from the hot end, using superheated 
steam. The control system obviates all necessity for flooding 
the superheater, and thus avoids two great dangers, that of 
wrecking the engine, and of strangling the superheater tubes with 
scale which must inevitably follow on flooding. The regulation 
of the control-valve permits any mean degree of superheat to 
be maintained within a narrow range, either at the highest 
safe maximum or down to little more than saturation tempera- 
ture. The desired point once fixed, and the controller locked, the 
remainder is, as described, automatic. 

The superheater thus effects the purpose (1) of a super- 



112 SUPERHEAT AND SUPERHEATERS 

heated steam-generator; (2) a controller of the superheat; 
(3) a boiler-water circulator or re-heater; and (4), in the sepa- 
rately fired type, a feed-heater and re-heater. As already stated, 
the superheater must be separately fired if water-tube or other 
boilers unsuitable for flue-fired superheaters are employed. 

Since about 1,200° to 1,400° F. is the maximum temperature 
to which any superheater can be subjected, it is obvious on first 
thought that a furnace-fired superheater is out of the question, 
for the furnace temperature must be at least 2,500° F. Separately 
fired superheaters, however, are made, in which this furnace 
temperature is diluted down by large volumes of cold air admitted 
between the furnace and the heater. This is so essentially a 
wasteful process that it must destroy quite a large proportion 
of the saving otherwise secured by superheat. In the control- 
lable system a water-tube and drum, or an elephant or French 
boiler, is built round the furnace. These water-drums, which 
may contain an hour's supply of water, are calculated to absorb 
one half or thereabouts of the furnace-heat, leaving the gases 
cooled to the required temperature to pass forward to the super- 
heater tubes. Only by such a method can a safe superheat be 
economically generated. The water-drums simply form a part 
of the feed-water system, taking feed from the economizer 
and delivering it to the feed-ring, or -main, at full boiler tempera- 
ture, or even hotter. 

The superheater itself, and its controlling parts, are similar 
to the flue-fired type, but, while the latter is suitable for a 
superheat temperature up to, say, 525° F., the separately 
fired superheater will give any superheat up to the safe maxi- 
mum. It is also applicable to old boilers which are too cramped 
in the flue-space for other types. In the case of a new steam- 
generation plant, we may say, approximately, that every sixth 
boiler may be omitted and replaced by a separately fired super- 
heater, which will do the water-heating and steam-raising work 



EXAMPLES OF SUPERHEATERS 113 

equivalent to the missing boiler, and will superheat the whole 
steam output. The efficiency of a superheater of usual self -fired 
type is only about 30 to 40 per cent. The gases leave the 
furnace at 2,500° to 3,000°, and they are at once air-diluted 
down to 1,200° F., or the hot gases are so baffled as not really 
to touch the superheater, which they of necessity must leave 
at about 650° to 750°. Air-dilution is the main safeguard 
relied on, and control is difficult, even with an expert fireman. 
A really controllable separately fired superheater should not 
require more attention than an ordinary boiler, and its efficiency 
should more nearly approach 70 than 40 per cent. It should 
not be less than 60 per cent, but this is possible only with the 
addition of the feed re-heater to absorb the surplus furnace 
temperature, and this is expensive; such expense, however, 
being recouped by the saving of every sixth boiler. Another 
advantage of the water-storage drums is that they exert a 
steadying effect where water-tube boilers of small water ca- 
pacity are employed. 

With regard to Figs. 20 and 21 it may be added that in the 
absence of an internal tube of water a superheater pipe may 
become overheated even when steam is flowing through it. 
Steam when superheated is somewhat diathermanous to radiant 
heat, so that heat from the tube itself radiates freely through 
the steam and passes across the tube into the opposite side. 
This cannot happen with the control water-core pipe, which 
absorbs the radiant heat and thus prevents the water-tube 
from attaining an excessively high temperature. 

In Fig. 22 are shown some of the forms of internal cast- 
iron cores, and also of control water-pipes, that have been 
adopted in practice for various diameters of superheating pipes. 



114 



SUPERHEAT AND SUPERHEATERS 






Fig. 1. 




Fig. 2. 



Fig. 3. 




Diagram showing Cross-sections of Various Types and Sizes of Tubes 
of Controllable and Accumulator Superheaters 

Fig. 1. — Loop-tube type, without 
water-control, but with heavy cast-iron 
gilled internal pipes, which act as heat 
accumulators and temperature averagers. 



Fig. 2. — " Field "-tube type, without 
water-control, but with heavy cast-iron 
filled internal circulating tubes, to give 
same effect as in Fig. 1. 

In Figs. 1 and 2 the outer tubes are 
weldless steel, 4 in. outside diameter X |-in. 
shell. 

Fig. 3. — Water-controlled Superheat- 
ers. Steel outer steam-tubes with internal 
solid drawn copper water-tubes, wherein 
the temperature of superheat is regulated 
and controlled by the action of boiler- 
and feed-water in constant and rapid 
circulation. These internal tubes pre- 
serve the metals of the steam- tubes and 
assist the boiler in the generation of steam. 

In Fig. 3 the outer steel tubes are 
weldless and 4 in. outside diameter, the 
internal copper water-tubes being lh in. 
outside diameter. 

Fig. 4.— Similar to Fig. 3. 

In Fig. 4 the outer steel tubes are 
weldless and 6 in. outside diameter, the 
internal copper water-tubes being 2 in. 
outside diameter. 




.Fig. 4. — 




Fig. 5. 




Fig. C. 




Fig. 5. — Steel outer steam- tubes with 
heavy internal cast-iron gilled water- 
tubes, which act as heat accumulators and 
increase the internal friction surface. 

In Fig. 5 the outer steel tubes are weld- 
less and 6 in. outside diameter. 



Fig. 6.— Similar to Fig. 5. 

In Fig. 6 the outer steel tubes are 9 in. 
outside diameter. 



Fig. 22 



EXAMPLES OF SUPERHEATERS 115 



Ferguson's Superheater 

This superheater, which is of English origin and introduced 
to America by the well-known steam-engineer Mr. A. Venning, 
is shown in Fig. 23 and is thus described in Power (August, 1907) : 

"This type of apparatus is suitable for giving high or low 
degrees of superheat, and easily adapted to all classes of boiler, 
especially those of the shell or return-tubular pattern. Lack 
of accessibility has been a very frequent failing in superheaters, 
and the Ferguson apparatus has been designed with the object 
of improving upon this too common fault, being so constructed 
that every joint can be examined with' the boiler in service, and 
any part can be removed for repair without disturbance or 
disconnection of the remainder of the ' superheater, which can 
remain at work without the missing portion. This is effected 
by the adoption of independent sections, a form of construction, 
moreover, which presents the further advantage that the sec- 
tions may all be of very moderate weight and the overhead tackle 
for use in removing parts may be of an equivalent lightness. 
According to the number of sections installed in an y particular 
instance, so will be the general degree of superheat obtained. 
From the position of the headers it will be plain that when 
removing or adding sections or otherwise dealing with them, the 
steam-pipe connections do not require to be disturbed. 

" But a point on which the designers perhaps lay more par- 
ticular stress is the manner in which the pipes are taken from 
the headers. They all rise from the top of these, thus obviating 
the danger of getting water into the superheater pipes, and 
affording the opportunity readily to drain it out from the headers. 

" This feature is likely to be a considerable safeguard against 
priming troubles and the choking of the small tubes with scale 
matter carried forward with the priming. This method of 



116 SUPERHEAT AND SUPERHEATERS 




IF 




EXAMPLES OF SUPERHEATERS 



117 



leading off the tubes possesses the incidental advantage that no 
joints remain in the hot gases, and this, in a small-tube super- 
heater, is a point of some considerable importance and tends 
to longevity of the tube-joints. 

" The tubes and headers, indeed the whole apparatus, are of 
mild steel throughout. Fig. 24 gives details of the header 
with the tubes in position, and a series of tubes is also shown 
separately. 






Fig. 24. — Detail of the Header and a Section of the Ferguson Superheater 

" The headers are so arranged that one end of each is connected 
to the steam inlet and outlet branches respectively, and as many 
sections as may be desirable can be fitted, their flanges being 
bolted to the headers. Each flange or pair of flanges takes 
three looped tubes, and, when removed for repair or to reduce 
the superheater surface, blank flanges cover the gaps left by the 
removed sections, these flanges, like the section flanges, being 
bolted to the headers. Similarly, in any emergency a section 
can be quickly replaced by a new one with a minimum of delay 
and stoppage. 



118 SUPERHEAT AND SUPERHEATERS 

" The application of the superheater to a return-tube boiler 
is shown in Fig. 23. 

" As shown in the figure, the headers or manifolds are placed 
on the top of the back arch, which is co structed of specially 
made fire-bricks capable of withstanding high temperatures, the 
blocks being designed so as to provide a series of narrow rect- 
angular openings through each of which a section of tubes will 
be suspended, the space being afterwards closed with asbestos 
so as to exclude air. Surrounding the headers there will be a 
brick wall forming a chamber, the top of which will be closed 
by a cast-iron cover and frame. This superheater is more par- 
ticularly provided for return-tube boilers, for which its form 
renders it easily applicable." 



CHAPTER XIV 

INDEPENDENTLY FIRED SUPERHEATERS 

It has already been stated that a superheater cannot safely 
for long at a time receive upon its tubes gases of full furnace 
temperature. Hence the usual fixing of the superheater at 
some place in the course of the boiler-flues where the gases have 
been reduced to a safe temperature. 

When the superheater is fitted with its own furnace the 
gases from the furnace are too hot to turn upon the super- 
heater, so that it is quite a usual practice to admit a deluge 
of atmospheric cold air between the furnace and the super- 
heater, thus reducing the gases to a safe temperature. Need- 
less to say, if, as may be the case, the weight of the gases is 
thus trebled, and since their final temperature must always 
be considerably above that of the superheated steam, there 
must be thrown to waste treble the amount of heat that would 
be wasted if the furnace-gases were not thus cooled down. 
Realizing this difficulty, Cruse applied a screen of water-pipes 
between the superheater and the furnace, thus practically 
converting the superheater into a flue-fired apparatus, which 
is certainly all that can be done with satisfactorily economical 
results. The preliminary water-screen or foreh eater is a 
reservoir or passageway for the boiler feed-water and is so 
proportioned as to abstract some of the heat from the gases 
before they reach the superheater tubes. There is no wasteful air 

119 



120 



SUPERHEAT AND SUPERHEATERS 



dilution; the superheater still has its internal water-control 
pipes, which may and usually are connected to the foreheater. 
In every way, therefore, this superheater is a flue-fired appara- 
tus, and the foreheater is a part of the general feed system of 




Fig. 25. — Independently Fired Combined Controllable Superheater and Feed- 
water Re-heater 

the plant to which the superheater is applied. Such an 
apparatus is shown in Fig. 25. 

In this design, prepared to fill Admiralty requirements, the 
superheater tubes D are placed horizontally and the gases 
from the furnace x are caused to pass through or between 
a bundle of tubes forming a feed-heater. Steam enters by 
way of the pipe C" and the distributor C, flows up through the 



INDEPENDENTLY FIRED SUPERHEATERS 121 

several coils, of which one only is shown in the figure, and 
escapes to the collector E and steam-pipe E '. The internal 
control-pipes of each coil of the superheater are taken from 
the distributor-drum K, which is fed through neck-pieces H r 
from the screen-drum H. The control water, usually in the 
form of foam escapes by way of the collector-drum N, which is 
open both to the drum P by way of the pipe R and to the back 
chamber of the water-screen. 

Feed to the screen comes in by F to the drum M, and the boil- 
ers draw their feed from the top drum P by way of Q. Steam 
escapes by way of the dome T and passes to the general steam- 
main and to the superheater, everything being under equal 
pressure throughout. The circulation of the water of control 
through the inner tubes of the superheater is effected, as already 
described under the head of the flue-fired superheater, by 
the agency of a superheated-steam inspirator if required in 
addition to the natural circulation. The superheater tubes 
may be vertical. The foreheater may be,; ; iade of any conve- 
nient form. The straight tubes in the example illustrated were 
to fill the Admiralty demand for straight tubes, but they have 
also been designed curved slightly and built up somewhat after 
the fashion of the Stirling boiler. Water-control pipes have 
the same pressure inside and out, so that they may be made 
light. They are now usually made of steel, but copper tubes 
have been employed on account of their smoothness and good 
conductivity, but steel appears to act perfectly well at a much 
less cost. 



CHAPTER XV 

THE PRACTICAL ECONOMY OF SUPERHEAT 

As regards the economy to be derived from the use of 
superheated steam it has already been said that Rankine's 
calculations in his book "The Steam-engine" did not differ 
much from actual results, though the coincidence was purely 
fortuitous. From 10 to 25 per cent may be given as the 
ranges of economy to be secured according to circumstances. 
With independently fired superheaters badly governed by 
air-drenching, as practised unfortunately in some cases in Great 
Britain, the steam economy will be very much better than the 
fuel economy. This is evidence of bad fuel use. The fuel 
economy should closely parallel the economy of steam. Super- 
heating is often applied in conjunction with other changes, so 
that it is difficult to apportion what is due to the different causes. 

So far as the author has been able to get results that can 

be declared reliable he may present the following figures of 

two runs of nine months each of a Yorkshire woollen-mill, where 

a Cruse controllable superheater was fitted to the boiler * In 

this example the water is taken at the natural temperature and 

heated up to 100° F. by the exhaust-steam of the engine. It is 

then at a suitable temperature to enter the economizer. It 

enters the economizer at that temperature and leaves at 230° F. 

It then enters the control-pipes of the superheater at 225° F., 

having lost 5° F. in passing from the economizer to the super- 

* See Proc. Inst. E. E., London, 1906. 

122 



THE PRACTICAL ECONOMY OF SUPERHEAT 123 

heater. It leaves the superheater for the boiler at 350° F. ; that is 
to say, the boiler working at about 120 pounds gage-pressure 
is at the same temperature as the feed-water which enters it, 
and the boiler is made strictly to act as an evaporator and not 
as a feed-heater in any sense. The temperature of the steam 
which enters the superheater is also 350° F., and the steam 
leaves the superheater at 495° F. It loses 30° F. in passing 
to the high-pressure cylinder of the engine. The engine is only 
a very ordinary sort of engine; it is not a Sulzer engine, which 
will stand very high superheat; it is only an ordinary factory 
engine. The steam leaves the high-pressure cylinder at 258° F. 
Between the high-pressure cylinder and the low-pressure 
cylinder there is a re-heater, which is fed by means of superheated 
steam. The superheated steam passes through the re-heater, 
superheats the exhaust-steam to 275° F., and the whole of the 
steam that passes through that re-heater goes into the boiler 
by way of the return water-control pipe of the superheater. Thus 
the only heat that the water loses, apart from radiation loss, 
is just the amount of heat it puts into the steam entering the 
low-pressure cylinder. As a result the I.H.P., which was 
formerly 367, has been reduced, doing the same duty and output, 
to 348 (this is due to the decreased resistance within the 
cylinder of the engine), and on the average of the same nine 
months of work in 1904 and 1905 there is a reduction in the 
coal of 200 tons on a total of 730. Originally, before the addi- 
tion of the stage heating of the superheater, 1'82 pounds of 
coal per I.H.P. hour were used; after the addition, 1*40 pounds. 
If these figures are worked out it will be found that this system 
of steam-raising, though only connected with a very small 
factory plant, gives very much better results than are obtained 
by large plants with the ordinary system of steam-raising, that 
is to say, by jumps instead of a steady range of temperatures. 
The following are the full particulars of the steam plant at 



124 SUPERHEAT AND SUPERHEATERS 

Low Bridge Mills, Keighley, which is the one referred to above. 
They cover the nine months, February to October, inclusive, of 
1904, without superheater, and the nine months, February to 
October, 1905, with a Cruse controllable superheater: 

Details of Plant. 

1 Lancashire boiler, 28 ft. by 8 ft. by 150 pounds working 
pressure. 

1 Cruse controllable superheater, 16 pipes, 200 sq. ft. steam- 
heating surface; 80 sq. ft. water-heating surface (copper 
control piping). 

(Added in January, 1905.) 

1 Green's economizer, 96 pipes. 

1 inverted vertical compound condensing engine; Corliss 
gear to both cylinders; Horsfall compound regulator 
governor; H.P. cylinder, 13 in.; L.P. cylinder, 26 in.; 
stroke, 3 ft.; revolutions, 100. 

Re-heater, between H.P. and L.P., heated with superheated 
steam taken by a branch from the main pipe. The steam is 
bloAvn through, and the exhaust is returned through the return- 
water collector of the superheater. 

During 1904, with "wet" steam, it was necessary to maintain 
the working pressure at the boiler at 150 pounds. During 1905, 
with superheated steam, the pressure was dropped to an average 
of 120 pounds. During both years the coal used was Yorkshire 
small slack, identical in quality and origin, the calorific value 
being about 12,900 B.T.U. per pound weight. Name: Rothwell 
Haigh-Smudge. Price: $1.12 per ton at the pit, and about 
$1.62 per ton delivered in boiler-house. 

The proprietor reports: "No additional cost in oil, having 
used the same quantity and quality during each of the two 
periods." 

No cost of repairs to superheater, boiler, or engine. 



THE PRACTICAL ECONOMY OF SUPERHEAT 125 

Observations during 1905. 

Average gage-pressure, 120 pounds (against 150 pounds in 
1904). 

Water: Average temperature feed entering economizers, 
100° F.; average temperature feed leaving economizers, 
230° F.; average temperature water entering controller- 
pipes of superheater, 225° F.; average temperature feed 
leaving superheater for boiler, 350° F. 

Steam: Average temperature steam entering superheater, 
350° F.; average temperature steam leaving superheater, 
495° F.; average temperature steam entering H.P. 
cylinder, 465° F.; average temperature steam leaving 
H.P. cylinder, 258° F.; average temperature steam 
entering L.P. cylinders, 275° F. 

Average vacuum : 27 in. 

Indicated Horse-power. 

Average for 1904, 367 I.H.P. (nine months); average for 

1905, 348 I.H.P. (nine months). 
Hours run: During nine months, 1904, 2,060 hours; during 
nine months, 1905, 1,900 hours. 

Altogether the I.H.P. for 1905 shows a lower average than 
that for 1904; the loads and outputs during the hours run were 
practically the same. 

Total coal used for all purposes for steaming and heating, 
for banking at nights and week ends and for power: During 
nine months 1904, 730J tons; during nine months 1905, 532 J 
tons. 

Coal used for steaming, heating, and banking: Average per 
week for both years, 3 tons. 

The steaming and heating are effected with "wet" steam 
through a separate pipe from the boiler, and not through the 
superheater. 



126 SUPERHEAT AND SUPERHEATERS 

Net coal used for power: 

Per Hour. 

1904 (nine months). . . 730^-117 = 613^ tons=0*2978 ton 

1905 " " ... 532J-117=415| " =02187 " 
Saving per hour =0*0791 ton = 26*55 per cent on power account. 

Coal used per I.H.P. hour, for power only: 

1904.. . . 613J tons = 1,374,240 lb. -(367x2,060) = 1*82 lb. 

(756,020) 
1905.... 415J " = 930,720" -(348X1,900) = 1*40 " 

(661,200) 

Coal cost per I.H.P. hour at $1*62 per ton delivered in 
boiler-house: 1904, 0*130 c. per I.H.P. hour; 1905, 0*10 c. per 
I.H.P. hour. 

Coal cost per I.H.P. hour (reckoned at pit mouth), $1*12 
per ton: 1904, 0*09 c; 1905, 0*07 c. 

All through the nine months of 1905 the boiler has been fed 
from the engine-pump, through the economizer, and through 
the controller pipes of the superheater into the boiler at the back. 
In the collector of the controller system of the superheater the 
feed-water from the economizer is amalgamated with circulating 
water from the boiler; the mixture passes into the boiler at the 
boiler temperature and partially already as steam. 

During the year 1905 it has been found advisable to reduce 
the length of furnace-grate from 5 ft. 6 in. to 4 ft. The boiler 
is hand-fired. 

The weights of coal given cover all the coal delivered to the 
mill during the periods mentioned, and used for all purposes — 
for power, for heating and steaming the mill, for banking up at 
nights and week ends. 

Assuming that the engine was driving an electric generator 
with an over-all efficiency as between I.H.P. and switchboard of 



THE PRACTICAL ECONOMY OF SUPERHEAT 127 

87 per cent, the coal consumption per k.w. would be 2' 157 
pounds per hour. This figure of course excludes the mill 
wanning and banking. Before alteration the figure would be 
2'804 pounds. 

If the figures be worked on the whole of the coal used, they 
become : 

For 1904 = 3*338 pounds per unit equivalent. 
" 1905 = 2764 " " " 

Economy on total account 17'2 per cent, due to the adoption 
of stage heating, fully heated feed-water, and superheat. 

In addition to the above economies there is the economy due 
to the reduction of I.H.P. per unit of factory output. Nothing 
is included for this, but it amounts to a further 6 per cent on 
power account. 

These results are somewhat remarkable, and they are results 
found by the mill-owner over a period of months when his out- 
put was greater and there was nothing altered except the steam 
temperature and the feed method, the superheater enabling the 
feed to be fully heated before it enters the boiler, the extra 
heat coming by way of the water-control pipes of the super- 
heater, as described in a previous chapter. 

The power apparently cost 26" 55 per cent less with than 
without the superheater on the basis of coal used, not including 
that used for mill warming. Where this is added the economy 
is of course reduced, proving that a very considerable part of 
the economy is due to the superheat at the engine. No specula- 
tion need be entered into in respect of the reduced horse-power 
with superheat, but it is not a matter of this one experience. 
It is thought that engine friction may be reduced by superheat 
if not excessive. 

It need hardly be said that neglect to employ correct devices, 
and especially a wrong method of firing a superheater and 



128 SUPERHEAT AND SUPERHEATERS 

improper and wasteful methods of control, may cause the fuel 
economy secured by superheating to be a long way below the 
economy shown by the steam-engine. This ought not to be 
the case. An all-round consistent economy should be secured. 
European engine-builders who build engines with drop-valves 
will guarantee a steam consumption as low as 9 pounds per horse- 
power hour. 

But steam economy is often largely vitiated by bad furnace 
practice, and it is poor commercial engineering to lay out capital 
in the purchase of a high-class engine for superheat when the 
economy thus undoubtedly secured is thrown away at the 
furnace. This is why flue-fired superheaters are usually better 
than those independently fired. 

But so many boilers cannot be satisfactorily fitted with 
superheaters that it is necessary to provide the other description, 
and a single large apparatus may superheat the output of a 
whole battery of boilers; and with a proper furnace, a fore- 
heater, and careful, rational attendance and firing, such an 
apparatus ought to approach a boiler in efficiency of heat 
utilization. 

In flue-fired apparatus of course the superheat is obtained 
from the gases before they have traversed the entire surface of 
the boiler and the boiler has been deprived thereby of a portion of 
its heat — just so much, in fact, as suffices to give the superheat. 
In round numbers if the boiler adds 1,000 heat-units to the 
water and the superheat is 100° F. and the specific heat be taken 
at the perhaps too low figure of 0*5, the heat given to the steam 
will be 50 units or 5 per cent of the total hitherto given to the 
boiler. 

Apparently to give superheat should require additional fuel 
to the extent in ordinary cases of from 5 to 10 per cent per 
pound of steam. But this does not necessarily follow, for many 
other factors come into play. Thus the engine demands very 



THE PRACTICAL ECONOMY OF SUPERHEAT 129 

much less steam and the heating-surface of the boiler is virtually 
increased, that is, its ratio per pound of steam produced is 
higher. Then in water-controlled apparatus the control-pipe 
forms a very efficient addition to the boiler heating-surface, and 
it may well happen that superheat will be given to the steam 
with little or no additional fuel, all the heat coming in any case 
from the chimney-gases which will pass away at a lower tem- 
perature. This is of course more apt to be the case where a 
boiler has been overloaded, for it will become more efficient 
when relieved of its excess of work. The economy of superheat 
per se could therefore only be found with reasonable accuracy 
by continuing the same rate of boiler working and using up the 
steam not now required by the engine in some other measurable 
way, thus determining the economy of steam at the original 
engine and that of the fuel used at the same old rate in the 
boiler. 

As already stated, with independently fired apparatus 
everything depends on the furnace design and on the means 
taken to temper the hot gases to a safe point for turning upon 
the superheater tubes. 

The best practice abstracts the surplus temperature by 
feed-water, converting the superheater into a flue-fired appara- 
tus in true character. 

The estimated steam consumption of compound condensing 
engines with a 27-inch vacuum is tabulated by Mr. W. 0. 
Webber for various pressures and a superheat of 100°, 200°, 
and 300° F. He takes a cut-off at f J, and J. (See Table VI.) 

The following brief abstract of the table for pressures of 50, 
100, 150, and 200 pounds will be sufficient. Intermediate 
figures can readily be interpolated. The table professes to give 
only ordinary results, not the extreme results of tests. 

Experiment, says Hiscox, serves to show that in general prac- 
tice about 8° F. of superheat will prevent 1 per cent of moisture 



180 SUPBRHB vr \M' SUPERHEATERS 

a1 ou1 oft r hen using saturated steam, I [e gives as the additional 
fuel nominally required for superheat the following percentages; 

i v ■! .■>• . oi Percentage of 

Supei luii Extra Fuel, 

, < ' F 5 per oent 

100° F 7 " " 

150° F II " •' 

200 c F i.'>" " 



Table \ i 

ESTIMATED STEAM CONSUMPTION OF COMPOl ND OONDENSING 

ENGINES USING SUPERHEATED STEAM in ORDINARY 

PRACTICE 



vo 
10 



iW fifth I'm Off 



I.O. 

IM'' 
IS , I 



1.00 

i l 10 
I | B I 

ie is 



i .; 30 
i ; | 
i | 08 
i i IQ 



1 1 60 

1! 01 
1 • V- 



One foui Oi Out >>u 



| . sr 
IS- 
IS s 

I" v 



16 r- 
16 .(- 
i. \4 



L3*88 

1 I IS 
1 I ,>0 



i | 10 

i ■• i 

i | 80 
13*39 



On* third Out off, 



i 
A 

ISO 
10 11 
IO0, 

10 I I 



16 ii 

10 si 

l , oo 



1 1 l/> 
I 1 SI 

18*31 

i | , ; 



1" .0 

i ;os 

ISM 






THE PRACTICAL ECONOMY OF SUPERHEAT 131 



Test of Foster Superheaters by Mr. A. C. Wood at Plant 

of the Maryland Steel Co., Sparrows Point, Md., 

December, 1906. 

The boilers serving the electric-power house of the Maryland 
Steel Co. being equipped with Foster superheaters, it was 
decided to run comparative tests for the purpose of demonstra- 
ting the value of superheated steam. The equipment consists 
of: 

Four 19 and 31X22 vertical Cross compound, automatic 
cut-off condensing engines, each direct-connected to 
one 300-k.w. D.C. generator; 
Three De Laval single-stage turbines, each geared to two 

100-k.w. De Laval D.C. generators: 
Two 14 and 24X14 Westinghouse single-acting, com- 
pound, automatic cut-off non-condensing engines 
belted to generators; 
Six B. & W. boilers 14 feet wide X 9 feet high, equipped 
with Foster superheaters. 

All the boilers were arranged for burning blast-furnace gas 
and were provided with auxiliary hand-fired grates. The 
superheaters were all of the Foster construction. Steam was 
supplied to the power-house through a 12-inch main covered 
with ordinary IJ-inch-thick magnesia sectional covering, not in 
the best condition. 

The first test was run with superheated steam; the second 
test was run with a reduced superheat, obtained by spraying 
water into the steam-pipe. The fact that there still remained 
some superheat in the steam under the reduced superheat con- 
dition proves that none of the water sprayed into the pipe reached 
the engines and turbines as water. The steam was something 
more than dry. 



132 SUPERHEAT AND SUPERHEATERS 

The electrical output of the station was measured and the 
water supplied to the boilers was carefully weighed. A sum- 
mary of the results obtained is given below: 

Higher Low 

Superheat. Superheat. 

Character of steam supplied, superheated . 119-6° 8-0° 

Steam-pressure, boiler-room 101 "6 lb. 100*0 lb. 

" engine-room 100*8 " 99*9 " 

Vacuum 22*1 in. 221 in. 

Decrease in steam consumption, due to in- 
creased superheat 16*82 per cent. 

Taking into consideration the fact that this is a comparison 
of a low and a higher degree of superheat, the result is very 
satisfactory. If, in place of the low superheat, the steam had 
been supplied to the engines and turbines of the quality supplied 
by the boilers without the superheaters, the saving would have 
been increased close to 20 per cent, says Mr. Wood, and this is 
probably very near the truth. In this case the fuel economy 
could not be known, for the boilers were supplied with waste 
blast-furnace gas for fuel and there were no means of knowing 
how much was used. 

In the absence of definite and authoritative knowledge 
of the specific heat of superheated steam it is customary to 
refer to the economy derived from superheating on a basis of 
the weight of steam consumed per horse-power hour. In this 
way the economy is made to appear much higher than really 
it is, for each pound of superheated steam contains more heat 
than a pound of saturated steam. Thus, let an engine con- 
sume 100 pounds of saturated steam. Let certain superheated 
steam contain 5 per cent more heat per pound than saturated 
steam, but let the weight used be 20 per cent less. 

Then when the heat consumption of saturated steam is 
100X100 = 10,000, that of the superheated steam is 80X105 
= 8,400 and the actual net economy is not 20 per cent but only 



THE PRACTICAL ECONOMY OF SUPERHEAT 133 

16 per cent, and this 16 per cent economy is what the fuel economy 
should approximate to or surpass, as it may perhaps surpass, 
under good furnace conditions and generally scientifically 
arranged plant. Fuel economy may be greater than steam- 
heat economy under some conditions, because the reduction of 
the duty of the boiler may place it in a better position to do 
its work well and economically. Such an advantage would be 
incidental, but could not be differentiated from the general 
sum of economy, though the cause may be surmised. This 
question of heat economy should be kept in mind, and though 
the specific heat of superheated steam may be in doubt, it may 
be assumed to be 0'60 for general practice and pressures in 
order to assist at a computation of the heat economy of the 
engine as distinct from the weight economy. 



CHAPTER XVI 

SUPERHEAT IN LOCOMOTIVES 






From what has gone before it will be gathered that super- 
heaters are somewhat difficult to supply satisfactorily to many 
classes of boilers. Either they must be placed beyond too great 
an area of boiler heating-surface to receive gases sufficiently hot 
to give adequate superheat, or they must be placed too near the 
furnace for the safety of the superheater itself. 

The locomotive is an example of the former condition, for 
the whole of the boiler heating-surface precedes any possible 
position of the superheater. In one case the inventor of a 
locomotive superheater replaces a number of small fire-tubes 
by one large tube in order that hot gases may get through 
in considerable volume to the superheater at the smoke-box end, 
and it may be here said that there appears very little prospect 
of obtaining satisfactory superheat without some serious struc- 
tural differences from the ordinary locomotive boiler being 
made. Thus Mr. F. H. Haughton of Richmond, Va., who has 
attacked the problem as a locomotive-builder, puts the front 
tube-plate of the locomotive boiler some three or four feet 
farther back, and in the space left vacant in the boiler-barrel he 
inserts a cylinder with tube-plate ends, and the same number 
of tubes within it that there are tubes in the boiler, but with 
the difference that they are so much larger as just to slip over 
the projecting ends of the tubes of the boiler. They thus 

134 






SUPERHEAT IN LOCOMOTIVES 



135 



serve to lead the gases forward through the superheater barrel. 
In this are two vertical diaphragms which divide the super- 
heater into three chambers in free communication with each 
other at the lower ends only of the plates. There are two 
dry pipes from the boiler, and they enter the top ends of 
each of the two outer chambers. The steam which thus enters 
passes down over the fire-tubes, turns under the edges of the 
diaphragms, and ascends amongst the tubes of the central 
chamber, whence it passes away superheated to the cylinders by 
a pipe opening out from the crown of the superheater. Better 




Fig. 26. — Fire-tube of Haughton's Superheater 

to secure good superheating the tubes which are in communi- 
cation with the middle chamber of the superheater may be, 
according to the inventor's patents, guarded from the water in 
the boiler by means of an internal pipe or guard, as in Fig. 26. 
These can be renewed if damaged by heat, and by changing the 
number of guarded tubes a certain regulation of superheat is 
possible. The guards serve to deliver hotter gases into the 
tubes which traverse the middle division of the superheater, 
and in this way they serve to overcome the chief difficulty of 
the locomotive boiler in respect of superheat. 

In another form of Haughton's superheater, Fig. 27, a long 
cylinder is let into the middle of the ordinary front tube-plate, and 
the ends of the tubes which are removed for a great part of their 



136 



SUPERHEAT AND SUPERHEATERS 



,0 to 




SUPERHEAT IN LOCOMOTIVES 137 

length to make room for this cylinder are expanded into the 
back end-plate of the cylinder. This cylinder serves to receive 
the superheater, which is a slightly smaller cylinder going 
inside the fixed cylinder and with tubes through it which, 
as above described, register with and telescope over the ends 
of the tubes which enter and project slightly into the fixed 
chamber. AVhen in place the superheater barrel is attached 
to the steam-pipe from the boiler and that to the cylinders, and 
only these two joints need to be loosened in order bodily to 
remove the superheater and replace it by a fresh one. The 
locomotive need not be many minutes out of commission to do 
this, nor must steam be let down to effect the change. This 
superheater forms also an excellent 'water-separator should 
the boiler prime, the diaphragms serving to prevent direct 
access of the wet steam to the cylinder-pipe; and if not dried 
out by the passage through the outer chambers, any water which 
may reach the bottom of the superheater chamber is there 
drained away. 

The application of superheat to locomotives now arouses 
very great interest in locomotive circles. 

Writing on the subject of superheat Mr. Vaughan of Montreal 
states that a Schmidt superheater was applied to a 4-6-0 simple 
freight locomotive of the Canadian Pacific Railway in 1901, 
and again the same apparatus was applied to two engines of 
the same type, but compound, in 1903, and the results showed an 
economy of 25 per cent in the case of the simple and of 15 to 20 per 
cent for the compound engines, as compared with engines of 
the same class using saturated steam. The first-named engine 
with superheat showed an economy of 18 per cent over compound 
engines of its class with saturated steam. 

A Cole-Field superheater was applied to a 4-4-2 passenger 
engine on the New York Central Railroad in 1904, and there 
were in December, 1906, no fewer than 197 engines in all fitted 



138 SUPERHEAT AND SUPERHEATERS 

with superheaters on the Canadian Pacific Railway, and 175 
more are on order. 

At the end of 1906 there were only 15 engines on all the 
United States Railroads thus equipped. 

Mr. Vaughan complains that the gases in the ordinary 
locomotive smoke-box are too cool for effective superheating, 
and draws attention to the use of the single large smoke-tube 
to admit a large volume of hotter gas to the superheater, but 
he does not seem to be aware of the principle of the Haughton 
superheater. 

Provision is made to shut off the flow of hot gas through 
the large tube when steam is not flowing through the super- 
heater, thus affording protection to the pipes. All the various 
types described by Mr. Vaughan at the Indianapolis meeting 
of the A.S.M.E. in June, 1907, depend upon an enlarged fire-tube 
or -tubes for the better provision of hot gas to the superheater, 
such tubes being sometimes 5" diameter, the pipes of the super- 
heater being 1J" outside diameter and -f^" thick. The 
general experience of the Canadian Pacific Railway is stated to 
be an economy for superheat of 10 to 15 per cent on freight 
service and 15 to 20 per cent on passenger service. There have 
of course been troubles. The gas-damper is found to be essential 
to the durability of the superheater tubes, as might be expected. 
Trouble was experienced where fittings and union nuts were of 
brass, as might very well have been foreseen. Made of steel 
these parts ceased to give trouble, and generally, after a pre- 
liminary elimination of weak details, maintenance has not 
proved expensive. 

As regards lubrication, it has been found necessary to 
lubricate both cylinders and valves, instead of only the valves 
as in ordinary working. The author remarks that he sees no 
valid reason to cease the continued application of superheaters 
to locomotives. 



CHAPTER XVII 

HIGH SUPERHEAT 

In all new movements there is a tendency to carry things to 
extremes, and by some engineers the use of superheated steam 
of very high temperature is regarded as an extreme. By high 
superheat is meant steam at 700° to 750° F. Now such steam 
cannot be passed into the working cylinder with safety, nor is it 
so employed. One Schmidt was closely connected with the 
"high superheat" so much talked of a few years ago. Now 
in the Schmidt system there was a secondary superheater 
through which the exhaust of the high-pressure cylinder was 
passed on its way to the low-pressure cylinder, and this super- 
heater was heated by the highly superheated steam on its way 
to the high-pressure cylinder. High superheat was thus merely 
a method of superheating the low-pressure steam in the process 
of letting down the highly superheated boiler-steam to a 
workable temperature. The Schmidt superheater also was con- 
structed somewhat differently from others, the steam making 
two passes through small pipes, one of them counter-current 
to the hot gases. 

Usually in small-tube superheaters the steam makes one 
pass through the loops of pipe, and it is argued by some that 
there is risk that the columns of steam may and do come through 
with unheated cores. As a remedy for this we have seen the 
mixing effect of the long cast-iron cores of the accumulator 

139 



140 SUPERHEAT AND SUPERHEATERS 

superheater, and of the centring pins of the core-tube of the 
Foster apparatus, the core-tube also preventing any possible 
formation of cold cores of steam. 

Velocity of flow is regarded as essential by many small- 
tube makers in order to brush the heat off the small tubes, and 
this idea has been carried to such an excess that the effect of 
the superheater has been to cut down the steam-pressure 10, 
15, and even 20 per cent by the throttling effect. Needless to 
say, such a reduction of pressure may countervail the economy 
derived from superheat. In large-tube superheaters the tube 
cross-sectional steam-passageway is made from- 1*25 to 1*50 
times the area of the boiler steam-pipe, and the length of run 
of the steam through the pipes is very great, the steam being 
turned over and mixed up at the end of each length of pipe 
in the header-box, thus insuring efficient superheating through- 
out. 

Cruse considers that time is distinctly an element in the 
superheating operation, and that steam superheated by long 
exposure may possess a superheat more permanent than 
hastily heated steam, the steam-molecules being more perfectly 
charged with heat. This so far is merely speculation founded 
on the belief that the superiority of long exposure to heat in a 
long pipe of many bends is not alone due to the elimination of 
core effect. 

There is still so much to be learned about steam and its 
properties that no theory which seems to fit with observed 
behavior in practice can be lightly passed over. But long and 
slow movement through the superheater pipes can be more 
safely carried out when water-control is employed. 

The small-tube superheater is of course much less costly 
than a water-controlled apparatus and has no equal durability. 
Water-controlled superheaters have worked for eight years 
with no apparent deterioration, whereas many small-tube 



HIGH SUPERHEAT 141 

superheaters have burned out in a few months. Though they 
burn out thus quickly, it is said by German engineers that 
superheat is so great a source of economy that even then a small- 
tube apparatus will give a considerable net gain. The actual 
gain given by superheat cannot, as said, be very well divided 
from the general gain, and the steam-user does not care to go to 
any expense or trouble in ascertaining these facts. He is satis- 
fied if in a period of months he effects a distinct commercial 
economy. Special tests do not much appeal to him, for they 
cannot represent actual every-day working. The author has not, 
therefore, introduced numerous figures of tests, for many of 
the published tests are purely academic and others are some- 
what colored. The figures from the Yorkshire factory are steam- 
users' figures, and as they are taken over parallel periods of 
nine months they fairly represent the truth and they show an 
economy quite sufficient to justify the installation of the 
apparatus. 



CHAPTER XVIII 

GENERAL REVIEW 

From all that has preceded, the student of the subject of 
superheat will gather that it bristles with difficulties and is in 
every application a matter of compromise. 

Briefly to sum up the matter, it may first be pointed out 
that a tube full of clean water may be safely exposed to almost 
any practicable temperature, for the water has a specific heat 
of 1; it readily absorbs heat from the surfaces it flows over; 
each cubic foot of water absorbs some 60 B.T.U. for each 
degree F. of temperature rise, while, when the evaporation- 
point is reached, each foot of water will absorb without further 
rise of temperature some 900 B.T.U. as latent heat. Not so, 
however, with steam. It already has acquired its latent heat; 
it does not readily absorb heat by mere contact with hot tubes, 
but the tubes must be considerably hotter than the steam. 
Its specific heat per unit of mass is but half that of water, while 
per unit of volume at ordinary pressures of 175 to 200 pounds 
it is not more than 1/200, and its heat-absorbent property is 
therefore small as compared with water, and it will not readily 
preserve the tubes, in which it is heated, from damage by 
excess of temperature. Then as regards the situation of the 
superheating apparatus, this again is a matter of great diffi- 
culty. In America the ordinary return-tube boiler and in 

142 



GENERAL REVIEW 143 

Great Britain the Lancashire type of boiler seem to be those 
which offer the best temperatures at the convenient point where 
the superheater is to be set up. Other boilers as a rule do 
not possess a place in which the superheater can be put that 
is so favorably situated. The gases have either passed over 
too much or too little of the boiler-surface, so that they are too 
cold or still too hot. 

Control of the temperature of superheat naturally offers a 
problem for solution. Connected as the author is with the 
only type using water-control in inner tubes, he is not un- 
naturally apt to favor that system somewhat. But such a 
system involves considerable additional first cost, and the 
engineer who would adopt it must use his own judgment as to 
this first expenditure and durability or a less first cost and less 
durability. He has ample choice, for there are the control 
systems by means of heavy inner cores to act by heat inertia. 
There are the excellent outer gills of the Foster superheater, 
which act also by heat inertia. The Babcock and some other 
superheaters protect the superheater tubes when idle by means 
of water which is admitted to the superheater; this then acts 
for such period simply as a part of the boiler, and the flooding 
water is drained out before the apparatus is again called to act 
as a superheater. Some superheaters, again, are safeguarded 
by means of a double set of dampers which regulate the propor- 
tion of hot gas admitted to the superheater tubes, while others, 
again, admit — wastefully and wrongfully, in the author's 
opinion — huge volumes of cold air in the case of separately 
fired apparatus. The Babcock Boiler Co. have a design for a 
water-screen in advance of the tubes of a separately fired super- 
heater. 

The engineer has plenty of choice, and he may even elect 
to follow the lead of some of the German engineers, who make 
little or no attempt to protect the superheater from destruction, 



144 SUPERHEAT AND SUPERHEATERS 

but rather build it up of the cheaper class of tubes and replace 
it as burned out, still, they say, realizing a commercial economy. 

As regards the more important protection of the engine 
by regulating the temperature of the steam sent to it, the risks, 
as pointed out, are apt to diminish as the number of individual 
superheaters combining to supply one engine is increased, for 
the steam temperature will be an average of many perhaps 
widely fluctuating tributary streams. All the foregoing sys- 
tems of regulation serve to control the temperature of the 
steam output more or less, and in addition, a thermostat device 
may be employed to spray water into the main steam-pipe, and 
this will act instantly in attemperating a too-high temperature, 
for a very little hot water will soon destroy all superheat, this 
disappearing as latent heat of evaporation of the water-spray. 
Such water-spray should always be of full boiler temperature in 
order that its effect in latent-heat absorption may be rapid. 

Needless to emphasize the fact that, given steam well super- 
heated and with no cores of half-heated steam, it matters not 
in what apparatus it has been superheated, the results at the 
engine will be the same. It is in the application of heat to the 
steam at the furnace end that the final economy must be looked 
for and secured. It is obviously indicative of bad engineering 
where a saving of 20 per cent of steam at the engine is accom- 
panied by a saving of only, say, 8 per cent of coal. This shows 
a fault somewhere. 

In quoting tests of engines with and without superheat the 
results have no bearing on the superheater. They merely indicate 
what an engine can do with superheated steam, and it were 
useless to present numerous tests, for they cannot do more 
than confirm the well-recognized fact that an economy of 20 
per cent of steam more or less may be secured as a general fact. 
Superheat, in fact, as such is not on trial. Engines are on trial 
and are now accepted or condemned on their behavior with 



GENERAL REVIEW 145 

superheated steam, and processes of superheating are also on 
trial, and time alone will show what system will ultimately be 
adopted. In the author's opinion there will for a long time be a 
field for superheaters of all the various types described, for con- 
ditions are so varied, financial circumstances have so much 
sway, and the personal idiosyncrasies of the responsible engineer 
bear so closely on the matter that the choice of a superheater 
for any particular plant cannot be foreseen. 

It will be noticed that the writer omits all reference to super- 
heaters with pressure-tubes of cast iron. These have done fair 
work in Alsace and elsewhere on the continent of Europe, but 
cast iron as a material of. steam-container construction is not 
a material much regarded by American engineers, and it is still 
less in favor with English steam-engineers, and for these reasons 
it is ruled out. 

It is customary to suppose that engineers in their designs 
endeavor to arrive at the point of maximum commercial economy, 
but it is certain that they do not in every instance. Apparatus 
is selected or rejected on quite other grounds. To take a familiar 
example, viz., the Green economizer. This apparatus was in 
common use in the textile area of Lancashire and Yorkshire 
probably fifty years ago. It was practically universal and was 
certainly never omitted from any professedly first-class factory. 
Coal in that area varied from 75 cents to $1"50 per ton (2,240 
pounds). In the South of England, including London, the price 
of coal was from $4*00 to $5*00. Yet until quite recently the 
economizer was scarcely employed in London and the South, 
where coal was expensive. Some of the electrical engineers 
even declared that it was useless behind a water-tube boiler, 
and where they had an economizer in place they would not 
use it. A few North-country men who knew its value used 
the apparatus for years before it became general. When it 
was finally adapted for electrical plants, it was only by copying 



146 SUPERHEAT AND SUPERHEATERS 

American practice, the Green economizer having first crossed 
the Atlantic and become established in America from the North 
of England before it obtained much hold in the South. So it 
is with other means to economy. Fashion, rather than calcula- 
tion or judgment or the dictates of commercial economy, will 
as often decide on the employment of superheat, its means of 
generation or control. The purity of the North-country water 
and the generally superior hardness of the chalk water of 
London no doubt had their influence in bringing about the dif- 
ference as regards the feed-heater, but no attempts at water- 
softening were made such as are made to-day, nor was the 
surface condenser attempted on land. 



CHAPTER XIX 

USEFUL UNITS AND DEFINITIONS, TABLES, ETC. 

The British (and American) unit of work is the foot-pound, 
or the energy necessary to raise a weight of one pound one foot 
high. 

The metric unit of work is the kilogrammetre, or the energy 
required to raise 1 kilogram through 1 metre. 

The kilogram = 2'2046 pounds and the metre = 3*2809 feet. 
Hence the kilogrammetre = 7*232 foot-pounds. 

Power is the amount of work done per unit of time. One 
horse-power is equal to the rate of work represented by lifting 
33,000 pounds 1 foot in 1 minute. 

One force de cheval = 75 kilogrammetres per second. One 
British horse-power = 1*0139 force de cheval or French horse- 
power. 

Indicated horse-power is the total work done by the steam 
against a moving piston in an engine = I. H.P. 

Frictional horse-power is the indicated horse-power of an 
engine when running unloaded = F.H.P. 

Brake horse-power = I.H.P. -F.H.P. =B.H.P. 

The thermal efficiency of an engine is the ratio of the heat 
converted into work to the total heat supplied to the engine. 

The mechanical efficiency is the ratio of the brake H.P. to 

the indicated H.P., or _.''_' . 

147 



148 



SUPERHEAT AND SUPERHEATERS 



The British thermal unit (B.T.U.) is the amount of heat 
required to raise the temperature of water 1° F. at or near 
39' 1° F. (or, as given by some, at or near 60° F.). 

The mechanical equivalent of the heat-unit is 778 pounds raised 
1 foot. 

The metric thermal unit is called the calorie and is the heat 
necessary to raise 1 kilogram of water 1° centigrade. 

1 B.T.U. =0252 calorie. 

1 calorie = 3*968 B.T.U. = 3087 foot-pounds. 

The specific heat of a substance is the ratio of the heat neces- 
sary to raise a unit weight of the substance one degree to that 
required for water, the specific heat of which is said to be 1. 

The following values of specific heat are useful for the en- 



Cast iron 01 30 

Copper 0095 

Ice 0-504 



Steel 0116 

Water 1000 

Wrought iron 0-113 



Const. Const. 

Pressure. Volume. 

Air 0238 0169 

CO 0248 0-177 

C0 2 0216 0171 

Hydrogen 3410 2-410 

Nitrogen 0*244 0'173 

Oxygen 0'218 0156 

Saturated steam 0305 

Steam at 212° F 0480 0346 

The difference between the specific heat of a gas at constant 
volume and at constant pressure simply represents the work 
done in overcoming the atmospheric pressure. 



The tenacity of iron and steel increases up to 500° F., but 
beyond this point they rapidly become weaker. Copper is 



USEFUL UNITS AND DEFINITIONS 149 

weakened by prolonged exposure to a temperature even so low 
as 400° F. 

Temperatures may be judged by the following color or 
brightness of a heated body. This is an approximation that 
may be useful : 

F. F. 

Bright cherry-red 1830° Dull red 1290° 

" heat 2550° Faint red 960° 

" orange ..... 2190° Orange 2010° 

" red 1470° Welding heat 2800° 

Cherry-red 1650° White heat 2370° 

Metals expand with heat a certain fraction of their length 
per degree Fahrenheit. The following are a few values: 

Brass ,- 000001047 

Cast iron - 0*00000616 

Copper 000000887 

Hard steel 000000695 

Mild " 000000672 

Tallow smokes at 430° F. and ignites at 570° F., but does not 
continue burning. It merely flashes. 

Water expands -^V °f its volume at 32° when heated to 
212° F. 



150 



SUPERHEAT AND SUPERHEATERS 



Table VII 
TABLE OF PROPERTIES OF SATURATED STEAM 



Absolute 
Pressure 
in lbs. 


Gage- 
pressure, 

Barometer 


Tempera- 
ture 

Fahrenheit . 


Heat-units per round 
from Zero Fahrenheit. 


Cubic Feet 
per 


*fl 


eight 
per 


per sq. in. 


at 29.922 in. 






Found. 


Cubic Tool. 








Total Heat. 


Latent Heat 






65 


50 3 


297 ■ 8 


120 1 S 


904 5 


53 


1533 


70 


55 


3 


302 7 


1206 3 


900 


9 


6 


09 





1643 


75 


00 


3 


307 ■ 4 


1207 7 


897 


5 


5 


71 





1753 


80 


05 


3 


311 8 


1209-0 


894 


3 


5 


37 





1 S02 


85 


70 


3 


316 


1210 3 


891 


3 


5 


07 





1971 


90 


75 


3 


320*0 


12116 


888 


4 


4 


81 





2080 


95 


80 


3 


323 • 9 


1212 7 


885 


6 


4 


57 





2188 


100 


85 


3 


327 


1213 S 


882 


9 


4 


36 





2290 


105 


90 


3 


331 1 


' 1214 9 


880 


3 


4 


16 





2403 


110 


95 


3 


334 5 


12160 


877 


9 


3 


98 





2510 


115 


100 


3 


337 ' S 


12170 


S75 


5 


3 


82 





2017 


120 


105 


3 


341 


12179 


873 


2 


3 


07 





2724 


125 


110 


3 


3 11 1 


1218-9 


870 


9 


3 


53 





2830 


130 


115 


3 


347 1 


1219-8 


868 


7 


3 


41 





2936 


135 


120 


3 


350 


1220 7 


866 


6 


3 


29 





3012 


1 to 


125 


3 


352 S 


12215 


864 


6 


3 


18 





3147 


145 


130 


3 


355 5 


1 222 1 


862 


6 


3 


07 





3253 


1 50 


1 35 


3 


35S ' 2 


1223 2 


860 


6 


2 


98 





3358 


1 55 


140 


3 


300 7 


L224'0 


858 


7 


•j 


89 





3463 


160 


145 


3 


303 3 


1224 7 


856 


9 


•j 


80 





3567 


105 


1 50 


3 


305 7 


1225 5 


855 


1 


o 


72 





3671 


170 


155 


3 


368 2 


1226 2 


853 


3 


2 


65 





3775 


175 


1(H) 


3 


370 5 


1 220 9 


851 


6 


2 


58 





3S79 


ISO 


1 05 


3 


372 8 


1 227 7 


850 





2 


51 





3983 


1S5 


170 


3 


375 1 


1228 3 


848 


2 


2 


45 





4087 


ISO 


1 75 


3 


377 3 


1229 


846 


6 


2 


39 





4191 


195 


180 


3 


379 5 


1 229 7 


815 





2 


33 





•1290 


200 


1 85 


3 


381 


1 230 3 


843 


4 


2 


27 





4400 


205 


190 


3 


3S3 7 


1231 


841 


9 


2 


22 





1503 


210 


195 


3 


385 7 


12316 


840 


•1 


2 


17 





4605 


220 


205 


3 


389 7 


1232 S 


838 


6 


2 


06 





4852 


230 


2 1 5 


3 


393 


1234 


835 


8 


1 


98 





5061 


2 10 


225 


3 


397 3 


1235 1 


833 


1 


1 


90 





5270 


250 


235 


3 


100 9 


1236 2 


S30 


5 


1 


S3 





5178 


260 


■J 15 


3 


104 4 


1237 3 


827 


9 


1 


70 





5686 


270 


255 


3 


107 S 


1 238 3 


S25 


4 


1 


70 





5894 


2S0 


205 


3 


411 


1 239 3 


S23 





1 


01 





0101 


290 


275 


3 


1112 


1 2 10 3 


820 


6 


1 


59 





6308 


300 


2S5 3 


117 t 


1241*2 


8183 


1 51 


0-6515 



USEFUL TABLES 



L51 



■A 

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oo t^ B ^i w « o o> » <e «d • ■ — o 


(0 r» oo 40) oi o *^ oi m " * "fl »<©h» oo 0>o« 




3 


Q '1 r- X CC O '- C 1 T- ■'. T 1 C- -C C T 1 C -C -T — 1 - i'", r - 

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i - 00 0) O — c i c i cc cc -r «c '.c i - 00 0> 40> O «- j 


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O « •- 1 - CC c;. iCCI Oi ■' Cl Ci «-' • - X ■' ' / 

i - 35 - c i - oi -/, / - v, - e . S c. o x i - <0 - « . ' i c 3 / 


— ifl (0 i - -/, K00HM " - - •' 14 (0 i - '/, 01 p — . • 


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3 


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j-Mj'.' i»i 


MI 


SSe^S98SRSSSSS§g$g8RS33Sg 



152 



SUPERHEAT AND SUPERHEATERS 



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USEFUL TABLES 



153 



Table X 

TEMPERATURE AND PRESSURE OF STEAM 

FOR EACH HALF-INCH OF VACUUM 



Inches of 


Alis. Pressure, 


Temperature, 


Inches of 


Abs. Pressure, 


Temperature, 


Vacuum. 


Lb. per sq. in. 


Degrees F. 


Vacuum. 


Lb. per sq. in. 


Degrees P. 





14 697 


21200 


15 


7-329 


178-96" 


h 


14451 


21115 


15* 


7-084 


177 44 


1 


14-206 


210 29 


16 


6-838 


175 87 


H 


13 • 960 


209 42 


16* 


6 592 


174 26 


2 


13715 


208 • 54 


17 


6 347 


172-59 


2£ 


13 469 


207-64 


m 


6-101 


170 86 


3 


13 223 


206 73 


18 


5-856 


169-07 


3* 


12 978 


205 80 


18* 


5 610 


167 23 


4 


12 732 


204 86 


19 


5 364 


165 31 


4* 


12-487 


203 91 


19* 


5119 


163 • 32 


5 


12 241 


202 • 94 


20 


4-873 


16125 


5§ 


11-995 


20195 


20* 


4-628 


15909 


6 


11-750 


200 • 95 


21 


4-382 


156 83 


6i 


11-504 


199 93 


21* 


4-136 


154 46 


7 


11-259 


198 89 


22 


3 891 


15197 


7* 


11013 


197 83 


22* 


3 755 


149 34 


8 


10 767 


196 75 


23 


3 410 


146 • 55 


8* 


10 • 522 


195 65 


23* 


3 164 


143-59 


9 


10 • 276 


194 53 


24 


2-918 


140-42 


9* 


10-031 


193 39 


24* 


2-673 


137 01 


10 


9-785 


192 23 


25 


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133 32 


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191 03 


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129 31 


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189 81 


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8 803 


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SUPERHEAT AND SUPERHEATERS 




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USEFUL TABLES 



155 



Table XII 
STEAM-CARRYING CAPACITY OF EXTRA-HEAVY STEAM-PIPES 

(E. H. Foster. Power Specialty Co., N. Y.) 







Pounds 


of Steam per 


rlouR at Velocity of 






8,000 feet 


per minute for dry saturated steam. 


Nominal 

Size 
of Pipe, 
Inches. 


Actual Inside 


8,500 feet 


per minute for 


steam superheated 50° F. 




8,950 feet 


per minute for steam superheated 100° F. 


in Square 
Inches. 


9,450 feet 


per minute for 


steam superheated 150° F. 


9,900 feet 


per minute for 


steam superheated 200° F. 




10,450 feet 


per minute for 


steam superheated 2£0° F. 




200 lb. Gage. 


150 lb. Gage. 


100 lb. Gage. 


50 lb. Gage. 


1 


0-71 


1,210 


872 


618 


362 


H 


1 


27 


2,000 


1,555 


1,105 


646 


H 


1 


75 


2,750 


2,140 


1,525 


894 


2 


2 


93 


4,610 


3,590 


2,550 


1,525 


2* 


4 


20 


6,610 


5,150 


3,660 


2,140 


3 


6 


56 


10,300 


8,050 


5,720 


3,450 


3} 


8 


85 


13,900 


10,820 


7,720 


4,520 


4 


11 


44 


18,000 


14,000 


10,000 


5,850 


4$ 


14 


18 


22,300 


17,350 


12,320 


7,230 


5 


18 


19 


28,610 


22,250 


15,800 


9,300 


6 


25 


93 


40,800 


31,600 


22,600 


13,210 


7 


34 


47 


54,600 


42,250 


30,000 


17,600 


8 


44 


18 


69,500 


54,000 


38,400 


22,450 


9 


58 


42 


92,000 


71,500 


50,800 


29,800 


10 


74 


66 


117,300 


91,500 


65,000 


38,100 


11 


90 


76 


142,800 


111,500 


79,200 


46,300 


12 


108 


43 


170,500 


133,000 


94,750 


55,400 


13 


132 


73 


216,000 


162,500 


115,500 


67,700 


14 


153 


94 


242,000 


188,200 


133,900 


78,600 


16 


176 71 


277,500 


216,200 


153,800 


90,500 


18 


226 • 98 


357,000 


278,000 


197,500 


115,700 



D. VAN NOSTRAND COMPANY'S 

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AUCHINCLOSS, W. S. Link and Valve Motions Simplified. 

Illustrated with 29 woodcuts and 20 lithographic plates, together with 
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BACON, F. W. A Treatise on the Richards Steam-engine 

Indicator, with directions for its use. By Charles T. Porter. Revised, 
with notes and large additions as developed by American practice; with 
an appendix containing useful formulae and rules for engineers. Illus- 
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BARRUS, G. H. Boiler Tests: Embracing the Results of one 

hundred and thirty-seven evaporative tests, made on seventy-one 
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Engine Tests : Embracing the Results of over one hundred 

feed-water tests and other investigations of various kinds of steam- 
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and diagrams. 8vo, cloth, illustrated $4 .00 

The above two purchased together $6 .00 

BEAUMONT, W. W. Practical Treatise on the Steam-engine 

Indicator, and Indicator Diagrams. With notes on Engine Perform- 
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and on Gas- and Oil-engine Diagrams. Second Edition, revised and 
enlarged. 8vo, cloth, illustrated net, $2 . 50 

BERTIN, L. E. Marine Boilers: their Construction and Work- 
ing, dealing more especially with Tubulous Boilers. Translated by 
Leslie S. Robertson, Assoc. M. Inst. C. E., M. I. Mech. E., M. I. N. A. Con- 
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K.C.B., F.R.S., Director of Naval Construction to the Admiralty, and 
Assistant Controller of the Navy. Second Edition, revised and enlarged. 
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BOOTH, W. H. Water Softening and Treatment, Condensing 
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CLARK, D. K., C.E. Fuel: its Combustion and Economy. 

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REED'S Marine Boilers. A Treatise on the Causes and Pre- 
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STILLMAN, P. Steam-engine Indicator and the Improved 

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TOMPKINS, A. E. Text-book of Marine Engineering. Second 

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VAN NOSTRAND'S Year Book of Mechanical Engineering Data. 

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WATSON, E. P. Small Engines and Boilers. A Manual of 

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ZEUNER, A., Dr. Technical Thermodynamics. Translated 

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